How come I get heavy periods at 17?

Common. There are many reasons for heavy periods and they can occur at any age. Common reasons at a young age are use of certain medications (aspirin, some herbals etc), not ovulating regularly, and some blood disorders. If they are consistently heavy, get a good exam and some blood work to evaluate them.
See a GYN. If your periods are too painful/heavy, see a GYN and get an US to detect problems like cervical lesions, infections, polyps, or fibroids. If you don't have any of these, you may benefit from hormonal control using contraceptives like the pill/patch/ring. The best option is a hormonal IUD (Mirena), that thins the lining and decreases your flow significantly. Some women even stop having periods!