Period for 3 1/2 weeks, what could be wrong?

For almost 3 months I had no period at all. I thought for sure I was pregnant although several home tests said not. I went to the Dr. to have bloodwork done, as an official check. Not pregnant. The Dr. then issued me a pill to help start my periods again. I never picked up the prescription because two days later my period started on it's own. The only problem is, my period lasted for almost 3 and a half weeks! I should be getting my period again in the next 3 to 5 days (if it remains normal) so I'm trying to hold off on another Dr. visit. Anybody have any ideas of what could be wrong? Thanks!