I'm having the same problem.. Except mine have been really light..
What kind of bc are you on? How long have you been on it?
I'm on orth tri-cyclen, have been for about 5 years now. But I just think my bodies hormone levels have changed.
I'd just take a test after your period ends to rule out the pregnancy factor. Take an early response one like clear blue easy or first response. They can detect pregnancy with hcg levels as low as 25.