If she is unspayed and your male cat is unneutered, then, yes, she is pregnant. How old is she? How long ago did you see them mating?
She can be spayed right away without any medical issues, if you don't want kittens.
Also, female cats can get pregnant right away after giving birth, which of course is very hard on their bodies. And male cats can still be fertile for a while after being neutered, due to sperm hiding in the tubes. So if you choose to let her have the kittens, have the male neutered right away so that he can't get her pregnant again after the kittens are born.
There are going to be a LOT more kittens in the future if you don't get them fixed. Cats usually have 2 or 3 litters a year, with an average of 4 kittens per litter. Do you want some assistance in finding a low-cost spay/neuter program in your area?