I haven't had my period for a while and than all of a sudden it comes back and it's lasted for about 3 months. what should i do?

Don't stress.: Always rule out pregnancy first. However, many things can affect the timing and regularity of one's cycle (e.g. Weight changes, diet, stress, etc..). Don't stress. Keep an eye on it for now. If the changes are persistent for more than 5-6 cycles, see your ob/gyn.

Anovulation?: Its possible that you aren't ovulating. Initially when you stop ovulating you can skip periods. But the lining of the uterus continues to grow and eventually you can have very prolonged bleeding. This is usually treated with hormones such as birth-control pills. See your doc to be sure and, of course, you need to make sure you are not pregnant. Best Wishes!