Very lightly spotting...but no period?!

I’m on bc pills and I’m supposed to get my period tomorrow. I’ve been on the pills for years now & I decided to skip my last two periods because the timing wasn’t going to work. Anyways, before I even reached my placebo pills this time around, I started to spot a little bit. I still had 2 days of active pills to take. Now I’ve taken 2 of the placebo and I’m still spotting. It hasn’t gotten heavier or stopped. I think I’ve had this happen to me before but why is this so if I have only taken ONE pill late? I’ve accidentally skipped a pill before and that usually doesn’t bring on a period? Anyone else experience this?