Why my period not coming back though I stopped Breastfeeding for 3 months (4mths pp)

Why is my period not back yet though I’ve already stopped Breastfeeding for 3months now? I’m currently 4months pp and I read that usually period would have returned. :( I just used pregnancy test kit to test and it was negative. I went to see my gynae earlier last month and he asked me to monitor for 6 months. What is wrong with me?