Put the "free kittens" ad in the paper, because they're coming! In fact, if her nipples are swelling, they're coming SOON! It's a little early in the year (kitten season usually begins in early spring) but of course a cat can come into season any time of year. You can probably feel the kittens if you lay your hand flat against her tummy. If you've had her since she was a kitten and haven't spayed her, she WILL get pregnant. In fact, she will get pregnant 1-3 times a year.

But Karen's right, she needs to go to the vet. If she has not been spayed, then chances are pretty slim that she's had any vaccinations. At the very least you will want her to be tested for any infectious diseases that can be passed to the kittens. You won't be able to vaccinate or spay her until the kittens are weaned (6-8 weeks of age) but please have both done as soon as it is possible. If it turns out she's not pregnant, please have her spayed and vaccinated now.