Confused~~

I have 2 boys, ages 6(August 2008) and 3(October 2011) well When my youngest was about 6~7months old I had a miscarriage. Shortly after that I got the mirena put in... no complications whatsoever with it. Well talking about another baby.. We recently got mirena out on February 17. Then on February 21 I had my period that lasted til the 28. Well I haven't had a period since that... I have taken several pregnancy tests and all are coming up negative but no sign of period.