Has she been wormed? If not, do her gums look pale? What about her siblimg's? Does her belly look a bit distended?
First- DO NOT give Pepto Bismol! It's toxic for cats. Keep clean water out at all times.
If you are feeding any milk, stop. That may fix it (some cats never become lactose intolerant, although they are usually on a dairy farm with an unending supply of milk. Others outgrow it very quickly.)
Worm her! Unless you are sure she is free of worms.
If you can afford a vet, it would be a good idea. I'm used to doctoring (and saving- one young vet called our place, "the farm where nothing dies that should!") barn cats, where a vet was out of the question. Feed plenty of broth, with some rice... it could be an upset tummy from new surroundings.
But it could also be early stage distemper, or a nasty stomach bug. Watch for sticky or runny eyes, sunken or filmy eyes (danger sign- dehydrated- vet needed stat!). Poop with bloody streaks, or gray and opaque liquid (can be salmonella) or almost pure liquid. The latter is also an emergency, as small animals can dehydrate very quickly.
Her normal body temperature should be 99.5 to 102.5. (Evercise and ambient temperature have effects.. so does a black cat lying in the sun!) Respiration should be 15-30 per minute. Just stand back and watch her breathe... count the breaths for 30 seconds. Multiply by 2.
If anything seems badly off, it's time for the vet. But cats get similar viruses that we do, and under similar circumstances. As long as you monitor vital signs (pinch the skin on her shoulder, hold for 2 seconds. Let go. It should snap right back. If it doesn't, she needs fluids. Call the vet!) and her attitude and condition, she may do fine at home.
Summerthyme