Still no BFP.. A week late..

So. I got off the depo in August 2016. I immediately started having a regular cycle. July and August followed that cycle as well. I have had a 35 day cycle since then. Spot on. Every month, AF comes right on cue. Well not this month. I am a week late as of today, but still getting a BFN. I have 2 other children. I tested at what would have been 6 weeks with my oldest, and got a BFN. But then a few weeks passed and a felt weird, so I tested again. BFP.. Went to the doc a few days later and they told me I was 11 to 12 weeks.. But I got a BFP with my youngest at 2 weeks after he was conceived.. (i was on the mirena, it moved so they took it out, tested 2 weeks later)

What could be going on? All the signs and symptoms are there..just not the BFP...