What Causes Severe Bleeding During Periods?

i am 22 yr old, had safe sex with my girlfriend. At the time of intercourse i was bleeding but it was a bit only.. but after days when my periods starts was bleeding heavily .. it's been 4 months and still when my periods came i had to change my pad 10 times in a day. Please help me.. am i normal ? is there any problem if i want a baby in future ?? please help me..