What Causes Heavy Bleeding With Clots And Pain?

hi im a 20 yr old female i had my menstural cycle in the last 2 months and on sunday night after using the bathroom i thought that it cam back cause i had some stomach ealier so i know it was comin but it started out in long blood clots and the pain is horrible and i keep seeing clots and my blood flow is very heavy i never had these problem before. i dont knw wat to do about the pain i rate it 9-10