What Causes Prolonged Menstrual Periods After Implant Insertion?

i had the implant inserted on the 12/15/10 and the week that i had it inserted i had my period for 8days then it stoped for a week and started again for 16days and for the month of january i had my period for 16days and this month i had it for 9days then it stoped for 4 and started again. ihave been in it for 17days now. is this normal