Rereading this it seems like her tummy is very painful to her. You said she doesn't like it when you touch it, and she won't lay on it.
It almost sounds like, if it's not worms and she has been spayed, that she has a very large abcess in her abdomen that is causing her alot of discomfort and also would explain why she is sleeping and not going outside, and also the fact that she is eating only a little bit at a time.
An abcess would be a very painful swelling that results from an infected wound. At this point the wound may already have healed, and was never necessarily noticeable or very bad/large. Cats often get these because the outside wound heals very well but then traps bacteria inside and this becomes an abcess.
Please take her to a vet to be checked. If she has worms bad enough to cause noticeable swelling, she is at risk for many health problems from it. If she has an abcess that large, it is also very dangerous. In short, I don't think your cat is pregnant and I do think she needs a vet very very very soon.
I tend to think she was spayed, or she would have been pregnant before now. Get her to the vet as soon as you can...if that is two weeks, that is what it will have to be.
For future issues, what works for my 23 y/o daughter (who is always broke!) is to save a few dollars out of every paycheck in the Rascal's vet fund. She was able to get him neutered by doing this, and he has always had the vet care he needs.
You've said yourself that you don't think she's just fat because of the sudden onset.
I have a credit card just for emergency vet bills and such, it might be something you should consider. And I bought Zissou health insurance for non-routine veterinary expenses.
Please consider that what is wrong with her might not wait two weeks. I know it's hard being broke, I don't have much money either, but is there anyone you could borrow from for a couple weeks?