K.P.
Yes. I actually asked this question to both my OBGYN and pharmacist. Apparently the antibiotics "break down" the BCP faster than your regular stomach acids meaning that the hormones are metabolized faster... meaning your body experiences the "crash" the initiates your period.
This also happens to me if I forget to take my BCP twice in the same month. I freak out about it and call the doctor and get the same answer every time!
However... you may want to go see your doctor b/c it's getting "worse" not lighter and it's lasting longer than the chemically-induced 5 days.