Eating Little, Vomiting, otherwise Normal

My cat Gatsby has got my very worried. I was out of town last week and had my mom look after him for me. During the week he vomited twice and at very little. I figured that it was because he missed me. I got back yesterday and fed him to have him throw it up later in the night. I tried to feed him today and he threw it up a few hours later and has since thrown up a clear liquid. He as been drinking water and acting fine otherwise although I can tell he has lost some weight. I read that some cats can develop an allergy to their food, so I have just now fed him some tuna (which he ate very little of) and he has kept it down so far, but I'm not very optimistic. Anyone have any ideas on what could be wrong?

If a cat goes more than three days without taking in nourishment, it can cause complications in the liver and become a dangerous situation.  There can be many causes of vomiting including bad food (either canned or dry), and the vets have several things they can give a cat to stop that vomiting and get some nourishment in to prevent liver failure.  I would suggest you get your cat to the vet ASAP, and in the meantime, get some calories into him.  Nutrical is a great thing to use, available at most feed stores and better pet stores.  It's basically vitamins and minerals and calories in an attractive tasty goop that you can rub on the roof the their mouth and they can then lick it off and swallow it.  Keep his blood sugar up one way or another, even if you just give him molasses in water. 

The last time I had an experience like this, the vet gave me a liquid called Amforol, which is an antibiotic that concentrates in the gut and wipes out any of the common food poisoning bacteria.  If that's the problem, results are fast and dramatic, as they were in my case.  It turned out that we had a bad batch of food, one that was later recalled.  I'm glad my vet used that as a starting point rather than running a battery of tests that would just show she was "sick" but not show why.

Anyhow, you can provide first aid to prevent liver damage, but a vet needs to see him and determine what's going on.  Good luck!