Implantation bleeding or Plan B side effect?

Hi girls!

Im on Day 36 of my cycle and my period is 5 days late. I took Plan B twice this month (Day 11 and 35) which gave me really bad headaches. The length of my cycles vary between 26 and 43 days.

I had some brown/almost black spotting that started yesterday accompanied by little bright red blood clots and some cramping. I still have a bit of the brown spotting today but not as much anymore. I don't feel nauseous and my breasts aren't tender at all.

I first started taking Plan B last month. (Note: I've never taken any form of birth control before that even though I've been sexually active since 2013 before that). My period last month started like this (brown/almost black spots) but it gradually transitioned into a normal period after the 2nd or 3rd day. However, this month it's already the 2nd day of spotting and it still hasn't transitioned into a full blown period. Before taking birth control, I've never had any spotting.

I took 3 pregnancy tests (Day 26, 33, and 36) which were all negative.

Is this implantation bleeding? Could I be pregnant?