The case of the black period. Help? I'm on birth control too...?

I'm 15 and I took Acutaine, which is an intense acne medication. And in order to take Acutaine you need some sort of birth control whether you're sexually active or not.

So I've been on birth control for about a year, maybe a little longer. And my period used to be really heavy and i'd get baaadd cramps. Like the blood would soak through my clothes. Yuck!

So now since I'm on birth control my period is light and it's great! I've been off Acutaine for about 7 months , but my parents are letting me continue birth control.

Well, my period used to last about a week before the pill, then after using it it lasted about 4 days.

WELL, recently my period lasted about 4 days and was heavier than usual, no big deal. My period ended, but then 2 days later my period started again and it's black and has blood clumps! EW!

It's still going on right now, and this isn't the first time this has happened....

any help? please don't say "ask your doctor" because that's a given.

haha