Period late for 34 days.

So I have a baby in June and almost immediately after having her my period when back on track like it never left. I only was able to breast feed for 2 months before I had to start taking medication that I could not breast feed on. Anyways, my period has been regular up until now. My last period was in November. I have taken multiple tests and none have been positive so I'm unsure of what to think. Should I call my doctor over this or does this just happen? I've never been this late besides for being pregnant.