What causes irregular periods?

I know this isn’t parenting related, but ever since I had my son (2 years ago) I’ve had problems with my periods being too heavy and lasting too long.
Eight months ago, I went and got on the birth control shot and it made it even worse. I was bleeding all day every day for 3 months straight. They begged me to come in and get my second shot and said it would stop my bleeding but it didn’t. That was 4 months ago. I’ve been bleeding all day every day for the last 7 months.
Anyone else have this problem?