Why Do I Have Excessive Clots And Clots When Using Tampons?

I've had a period for going on 6 weeks now. I have an appointment for Monday, but just noticed a common thread in the whole thing. I'm having HUGE clots when I try to use a tampon. It's the only time I have cramps at all, and the main time I have excessive bleeding and clotting. What could the connectino be to tampons and clots?