I Am 19 Years Old And Have Had My Period

Question : I am 19 years old and have had my period regularly since I was 11. I have never had issues with spotting between periods until recently. In August, I got very sick with mono and had to move twelve hours back to college. I started bleeding and this lasted for two weeks before my period, then I got my period, and the bleeding stopped. I went to the gynecologist and was told it was probably just stress-related. Figured she was right because the next month came and went as normal. However, in October, the same thing happened - bled for two weeks, had a short period, and it stopped. Now, a week and a half after my period stopped, I'm spotting AGAIN. It started out as kind of a pale brown, but now it's dark dark red, almost black. I am having some pelvic pain on the left side and some cramping, but nothing severe. I am NOT pregnant, and I have taken the same birth control regularly since I was 15. I have always been stressed due to school and I have never bled in response to that, so I doubt that this is really "stress related" since it continues to happen. What's going on?