What Causes Heavy Bleeding During Pregnancy?

hi my name is shawanda and I think Im about 17 weeks pregnant. I haven't been to the doctors yet because I havent told my parents yet. But, in january I got my period around tha 20th or 21st,I had unprotected sex in february and missed my period that month. It came back in the beginning of March and has been coming back every month since then and lasts for about 5 days. But this month,it came back yesterday and today Ive been noticing alittle much of blood clots.Ive been through about 6 pads in a day. Im scared I might be having a miscarriage...Please help me