please help

My cat Oliver has not been feeling well for a couple days. Monday or Tuesday I gave him old food and he had diarrhea and vomited once. I realise it was my fault and I put him on wet food to clear up his diarrhea. 
He never once acted like he was sick, which is why I said it was the food. He's been playful, waking us up, silly, and naughty (at the bunny) like usual. 
About 20 minutes ago he vomited again. It looked like light colored gravy with nothing in it except a couple hairs. I was thinking he might have one of two things, he might have a blockage?? And he might be showing intolerance to the wet food we feed. 
I feed grain free dry food and the wet food is whatever. I was still trying different ones but his favorite seems to be Friskies turkey slices. I do not live close enough to a pet supply store to get non-store brands for wet. His dry is PC Nutrition First grain free. Which is not the best, I know, but it's what we have available. 
Today we're going to get hairball remedy just in case. 

Oh, one important thing, he's due for his second dewormer pill on Saturday. We know he's been affected by worms pretty bad. When we got him (Nov 4 '11) he was tested negative for feline leuk and flv and was given dewormers for his always-in third eyelids. 
Could he be getting sick because of a second wave of worms? And can we give him his second pill 2 days early? Thanks!!

 Thanks everyone, I'll call the vet. I honestly don't have the funds for the vet right now. I feel bad about that but after having my dog to the vet, euthanised, and cremated al in one student loan I am literally living on what I make a week- which is barely enough to eat on. I got him a lick stick to make sure everything is going through. I'll give him the de wormer tonight as well. 

And I forgot to mention, yes he is using the litter fine. There's nothing odd about his behavior at all. Other than the vomiting, which occurred this morning. He's been actively involved in our activities and he's sleeping no more than usual. 

Thanks everyone!

 @nebula-  I think I'll hold off on the shot for now bc I don't want to over run his system on de wormers! But if they persist then I'll look it up :)

 @ferelvr- thanks. My dog meant everything to me. I'm still depressed :(  The vet and the envelope for the pills said to space them 3 weeks apart! So he was technically due for his 2nd tomorrow but I gave it to him today. SO I'll give him the last one in 15 days instead of 21? The brand is Milbemax. 

 @MediCATe- He hasn't been acting sick or vomiting since when I originally posted this. He has the runs still. Not explosive as when I first noticed it and certainly not as stinky but I watched him poop today to see. Even though I didn't take up food and he's been eating the same, should I still limit his amounts?

 @Auntie- I don't think he touched any plants or chemicals because the only surviving plants I have left are an aloe vera and another type of succulent and they're both really tiny he doesn't even know about them. His dry food is grain free but I agree something in the can food might be upsetting him. He has never been more than 2 hours, let alone 8, for food!! Even if his bowls are empty he comes bawling for more- Matt gets up in the night to fill it. I must have put it across a little wrong, I didn't "put him on" wet food absolutely. I meant, he always gets wet food, every day. But the first day when I "put him on" wet food I meant I took up his dry food so all he could eat was wet. I was initially concerned with dehydration and looked it up. [I was going to suppress food altogether bc that's what I used to do with my dog, so I'm glad I looked it up!] 

 Thanks for your help and concern guys! Ill definitely keep you updated to his progress. I hope it's as simple as his food. :/  

Done and done :) Thanks a lot ^.^

ok so I am not with-holding food. I would never do that unless I was instructed to do so prior to surgery by a licensed vet. Also, Milbemax is what my vet gave me. I seen the vet. I'm not just here playing with medication I found in a corner store. 

 Ok last night I went to sit by my bunny before bed (she no longer has free run bc Oliver needs to learn his manners) and when I did Oliver jumped out of his sleepy sleeps and ran over, dropping a small solid poop. Never have I been so happy to see poop on the floor! Then I cleaned the litter box, I am happy to say my baby is all better! Matt cleaned the box the night before and he's obviously not as concerned looking for solid poops as I was (he even went as far last night to say, after I said Oll was better., " Oh, he still has diarrhea? I thought that was only for a day".. -_-" ) 

 So, I'm sorry about snapping in the last post by me. It seemed nobody had actually read thoroughly and were accusing me of playing with fire basically. I would never. I know there are a lot of dumb ignoramus' out there but I am NOT one of them. I simply said I wanted to hold off the vet for now. I'm also not one to rush to the vet at every little thing, that doesn't make me a bad owner. It makes me appreciate and recognize serious things more readily, bc I can differentiate between serious and non serious things. 

Yep, as East as you can get in Canada. Maybe it's an Atlantic Canadian thing? I don't know, here's the link: http://www.ah.novartis.com/products/en/milbemax_cat.shtml