It could be due to the nursing (even sporadically if your own body is very sensitive to nursing... some women are, others like me are the opposite and get fertility back despite EBFing to the rules. 6 weeks, both times. it sucks lol)
Or it could be due to other problems... such as a hormonal imbalance...
I would talk to a doctor about it as they would be able to diagnose any hormonal problems and fix them fairly quickly and easily in most cases.
Artist, photographer, stay-at-home-mom and Marine wife. Mom to 4; a boy and three little girls.