Cat has been vomiting but seems otherwise perfectly healthy and is ravenous!!!!!

We have a weird stomach issue - my 8yr old Siamese boy is not keeping his food down! The whole thing started Monday night, when he all of a sudden couldn't settle and seemed in pain (body language etc.). Plus, he wasn't interested in his favourite treats, which means there is something seriously wrong!! I gave him some Buprenorphine orally that I still had, but he brought that up a little while later (or, he brought up some clear liquid, which I'm pretty sure contained the Buprenorphine). At that point in time, I thought that was the only time he'd vomited; however, the next morning I did find a pile of vomited food, which must have been from that same night and which would explain why he seemed nauseous (lip licking etc.). He seemed much better the next day - back to normal, actually - with a good appetite and a good attitude, so I thought it was just some sort of solitary stomach issue. Unfortunately, he vomited again during the night....and seemed ok again the following day!!! That was Wednesday. I also had a good friend of mine, who is a vet, come round to check him over and she palpated his stomach and said there was some poop and some gas in him, but not masses of stool which could have made him all constipated and caused the issue (he does have a history of very firm stools, typical Siamese, and is on Lactulose and Miralax - alternatingly). He had also not produced a lot of stool since the problem started, and while my friend was here, he went to he toilet und VERY LOUDLY passed gas!!! Which I'd never heard before!! So he really must have been very gassy indeed....Well I really hoped that was it since, once again, he seemed wonderful in and of himself, purring, rubbing, cuddling etc., good energy levels....Unfortunately, that evening, he vomited up his food YET AGAIN - pretty much undigested, just like the other times!! Once he was done, he jumped right back into my lap and purred his little head of - and showed an interest in MY dinner, so was clearly not nauseous, again!! Nothing happened during the night, but my husband heard him making the upchucking noises in the morning (I was still in bed) and he had just brought up a little clear liquid - which I thought was odd, but just a little while later, I found a small pile of chewed pieces of grass in some more liquid, so I'm almost 100% sure that he ate that grass, brought it up and the little thing of liquid we found was came after the initial grass vomit - which actually reassured me somewhat because I thought it made more sense for him to bring up grass than just some liquid, out of the blue!! Anyway, I then gave him just a very few pieces of ID kibble, which I still had from cat sitting my friends' cats, who eat that (it's a gastrointestinal food) and my vet friend had told me to give him some Pepcid AC, which I did (in a couple of Pill Pockets). She had also said to fast him, so I didn't give him anymore - even though the poor cat was RAVENOUS!! As far as I know, that stayed down as I didn't find any evidence of vomiting; I then headed out to work and left he poor hungry cats at home. When I got home from work, he was desperately hungry and following me around, demanding food....so I broke down and gave him a very small portion of that ID kibble, along with a couple of Pill Pockets in which I had the second dose of Pepcid. Everything seemed honky-dory, until I went back inside from the deck - and stepped into a small pile of, you guessed it, ID kibble - and again some grass, because my husband didn't know that he wasn't supposed to eat any and had let the cats go into the yard, where we have grass...Needless to say, I am devastated and had a little cry!! I love this cat so much, and I just don't know what's going on - he seems so happy, and he just doesn't udnerstand why I won't feed him!! But if he continues to not keep anything down he'll starve!!! He's a big boy of over 13lbs, so a little weight loss is not a problem, but obviously in the long run, this cannot go on.....We have had blood work done in February before his dental, all was good, and I just took x-rays of him last week (I work at a vet hospital and we got new digital x-ray, so I took the cats in for some free practice x-rays), that was all good, too - some stool but nothign out of the ordinary or masses or anything!!! I guess I will take him in tomorrow and at least do a pancreas SNAP test to see if it's a pancreas issue, but from all I know, they are NOT ravenously hungry when they have pancreas problems!!! Has anybody ever had a similar situation and can share their experiences / advice??? I am slowly going insane....Thanks in advance and for reading this very long essay!!!

His blood work may have been great in February, but this is now July...things can change in that amount of time. 

I honestly don't know much about the differences between xrays and ultra sounds, but DO know that there are differences.  I thought xrays showed more bones, and ultra sounds showed more soft tissue stuff (things like if there were a mass present), but what do I know...I don't work in a Vet's office, that's for sure   If he had a hairball stuck somewhere down the line, would that show up in an xray? 

The only similar experience I can share is that I have a cat who throws up that frothy stuff (no food), and in her case, it's over acidity because she doesn't eat enough.  The way we "fix it", is with injectable Famotudine (pepcid a/c) and try to feed her small meals all throughout the day and night. By small I mean maybe .5- .75  oz 6 times a day.  She only weighs 4.5 pounds.  Then during the night I keep freeze dried raw nuggets in a baggy under my pillow and she'll come and wake me up for some every few hours

Hmm...

Could it be a bowel obstruction?  How long after eating is it coming back up?

Is he able to keep water down?

Not always is a bowel obstruction obvious on an Xray... it also depends of the quality of the Xray and also what the blockage consists of. Can your vet's office run a Lactic acid level??

Since it seems to have started to suddenly, and he is not acting sick in general, but hungry,,, (but as a blockage progresses, there will be pain)

You may not know of anything that he could have eaten that would cause an obstruction, but it could have been

some time ago, and somehow it just now started blocking.  (Ask me how I know!! )

Hope you will find an answer soon!