What Causes Heavy Periods?

hi i am a year after birt and was on a light pill called cerazzete.i am still breasfeeding and got no period untill about 4 weeks ago when i flew to another country.i started spotting for about a week then when i flew back i got a light period.that cleared but now three days later i got a really heavy period.wanted to know what you would suggest.for now i am not taking the pill.