I agree with Ghilly. Pits are very stoic and will endure incredible amounts of pain without flinching. Please get her to a vet ASAP. And let us know!
We can not even begin to hazard a guess as to what might be wrong with your dog. If a dog is sick enough to stop eating, then a trip to the vet is DEFINITELY in order. Please do not wait any longer. Make an appointment for her ASAP, in fact, since she has already stopped eating and keeps to herself, I wouldn't wait until tomorrow, I would take her to an emergency vet TONIGHT. Sometimes, depending on what the problem is, a few hours can mean the difference between life and death, and since you have no idea of what is happening with her I wouldn't take any chances. Please let us know what happens.
Ghilly