Are There Any Changes In Menstural Cycle During Breast Feeding?

I am currently breastfeeding my first child, who is now just over six months old. He has been eating more than ever, despite the subtle introduction of solids (pureed carrots, etc). My periods returned after three months and have been normal up until this point. I am now a week late but I have taken two negative pregnancy tests in the past week. Other than fatigue and no period I have no pregnancy symptoms, so my questions is that is the change in my period most likely due to the breastfeeding, and at what point should I consider getting a blood test done for potential pregnancy?