Why does my cat have a round, hard stomach that comes and goes in severity?

First off, I want to say that I AM taking my cat to the vet. He has an appointment for Friday. However, I was wondering if anyone knew what could be causing a round, hard stomach that sometimes goes down in the roundness and sometimes gets softer, like it fluctuates. I had been giving him dry food, but I recently switched it to wet food, because he wasn't eating (this was after the stomach thing appeared). I've also been giving him milk, because he won't drink his water, and I want him to get as much hydration as possible. He's been quite lethargic lately. Sometimes it feels like he gets hot, but then he starts acting normal after the heat subsides some (being more playful), so I was thinking that could be a fever trying to get rid of the disease. He's still using the bathroom normally, if anything, he's using it more. He pees a little bit more, but it's only, upgraded from 1-2 times daily to 3-5 times a day with 1-2 defecation litter visits (which is normal for him). I think he may also have high blood pressure, because his pupils look a little dilated, but at this point, I'm reading everything online and thinking he has all these symptoms, so I'm making myself paranoid. Anyway, any ideas?