Forgotten pill and bleeding!

Hi everyone. I've been on Minastrin24Fe for about 3 weeks now, and forgot to take a pill last night for the first time. I just ended up taking it this morning, but was wondering... I started bleeding a little bit today- thick, darker blood and not too much of it. Just a little. Why did this happen? Is something wrong or is this a type of spotting that is purely coincidental? Thanks!