S.B.
Well, while rare, it is possible for a vasectomy to fail. If you are normally very regular with your periods, take a cheapo dollar store test just to be double sure. If it's negative, you could just be under a lot of stress that is causing your cycle to get wonky. It sounds like you have been dealing with abnormal pap smears, which I know from experience can be a stressful thing! Try to relax, and if it would help you to feel better give your doctor a call.