What Causes Heavy And Continuous Bleeding?

i have been bleeding very heavy the past three months on my period. i bleed 3 out of the 4 weeks in a month. i am bleeding so heavy everyday that i have to wear a tampon and a pad at the same time. and if i do stop bleeding and have sex i start pouring blood again. what do i do? should i go immediately to a doctor or is this normal? i am 21 years old. not overweight and i am gay so pregnancy is not an option.