Well if she is over 5 months, not spayed yet and allowed outdoors or in contact with intact males, you can bet that she is pregnant. Please keep her inside, away from intact males and spay her asap. If she is pregnant to the point where you are really able to tell, then it is probably too late to spay/abort so search around here for the threads about having pregnant cats and what to do. Put her on a high quality kitten food asap. Also start setting serious money aside because there is always the risk of illness and trouble delivering. C-sections are really expensive especially if done in after hours at the vets office. Plus all the extra kitten food and litter, and then the series of vaccinations for all the kittens and spay and neuters for the whole family.
Good luck and HOPEFULLY you are lucky and she isn't pregnant but just has worms or something