Well, it's hard to say. The obvious question is if you are sexually active and if so, if you've taken a pregnancy test. Do so if the answer is yes to being sexually active. Otherwise, this can happen. My thyroid was functioning slow and I had very long cycles (which is what a late period really is). I got my period every 3 to 4 months! But I'd have pms throughout that time waiting for it--- it was a real pain. Talk to your doctor if you go three months without a period. good luck