What Causes Heavy Vaginal Bleeding After Taking Pills For Menstrual Problem?

I'm 19 years old. I started taking a pill to help regulate my periods when I was 16. It did, but it didn't help the heavy bleeding or the pains at the beginning of periods. I changed pills about a year and a half ago and was asked to run the packs together. Recently I've bled three times in around a month after taking the packs non stop for around 6 months. Is this normal? The times in which I've bled have usually lasted a week each. Can you help me? Many thanks