Rash on bum - When to be concerned?

The last few days my cat, Ichi, had been dealing with a case of diarrhea. We took him to the vet yesterday to get it checked out and make sure there wasn't a bigger problem. The vet really didn't give us any answers as to why he was having besides it being a dietary change or something he ate. I know I should have taken that as a relief but there hadn't been any change to his diet besides giving him a little more canned tuna (his special treat) a little more than usual. And he just doesn't have an interest in human food and we hadn't really left anything out for him to get into. 

Anyways, all the irritation from the bathroom problems and the licking the area resulted in him actually getting a really bad rash down there. They shaved and cleaned up the area down there, and the last day I've been trying to keep it clean with baby wipes and keeping extra strength healing ointment (aquaphor, is what the vet recommended) on and around the rash to help it heal and stop him from licking more.

I just am not really aware of the healing phases of a rash and I don't know if it's supposed to look like a really dark, almost brown area around it? (I know it sounds like its something fecal around it, but it's not cleaning off and he also hasn't used the litter box for that since he got home from the vet yesterday)

Another somewhat silly question someone could maybe be able to help me with is when he should be going to the bathroom again? He has had a really decreased appetite, and will still eat and drink but doesn't seem to be doing anything but going pee. 

Hi there,

I am really sorry your cat is sick and it sounds like your vet did not do a proper examination. I am concerned that he has not used the litterbox since yesterday and is not eating well. Did the vet give him any medication? I think taking him to another vet might be a good idea. He probably should have a stool test done. 

A bland diet of plain boiled chicken is often good for cats with diarrhea. 

What kind of food does he normally eat? He may have a food intolerancy going on.

He's been eating Purity One Smart Blend for about a year now. 

At the vet they gave him some anti nausea medication, and gave us antibiotics he's supposed to have twice a day for a week. 

I just kind of feel like when I spoke the vet I didn't give him enough to be concerned about? Ichi has been acting like nothing is wrong, hes not lethargic and his attitude and energy level has been the same throughout all of it. He also hadn't been vomiting. 

He has used the restroom to go pee, but it doesn't look like he's done it for anything else, but I'm confused cause it looks like he has since there is a kind of fecal matter around his bum? But I feel like that could be a scab from the rash, but it doesn't feel like a scab?

I'm going to consider taking him to another vet though. 

Ok good he has used the litterbox to pee that was mainly why I was concerned. How much has he been eating? I would recommend trying to get him to eat the boiled chicken it should be easier on his stomach. It might have passed now and he just does't have much in his system. 

One thing to consider is that it is his diet that has brought about this stomach upset. Cats can easily develop an intolerance to their foods, especially to grains. Cats are obligate carnivores and the purina one dry food are made up mostly of grains, which are harder for cats to digest. If you would like to learn some more information on feline nutrition take a look at www.catinfo.org which is a great site dedicated to the science of feline nutrition written by a vet.

He's been eating...maybe 1/4 a cup of food a day? We've always free fed him and I'd say before he ate anywhere from 1/2 cup to 1 cup of food a day.

I'm going to look into that site though and look into an alternative diet for him. 

Thank you so much for all the advice.