What Causes Inconsistencies In Periods?

i have a 15 month old boy. i breastfed for 13 months. at my 6 wk postpartum checkup i had a implanon inserted into my arm for b.c. i just got my first period the week of oct 1. i got it that mon, it ended the next monday. i got it again less than a week later wasn't too bad, just normal to lite bleeding. it stopped for about 2 days, long enough to have sex one time. then i have it still, and now it it MEGA HEAVY. it is so gross and yucky and interferring with life. is this normal after stopping breastfeeding. (oh, forgot to mention i was periodless for 14 months plus the time of pregnancy) what can be done, and is this a major problem? could it be dangerous, or is it just annoying?