Why not take her to the vet to see for sure whether or not she is pregnant? If she is, the vet will be able to tell if the kittens are too large for her to deliver thhem safely. If there is a danger to her, it is possible to have her spayed right now, even though she's pregnant. Of course, she won't have the kittens, but you won't have to worry about her getting pregnant again in the future either. If she is really small, there's a chance that she could die trying to have the kittens if they are too large for her to deliver easily and safely. Only a vet can tell you for sure.
Ghilly