Unsure and concerned

Can someone please explain this, it hasn’t happen to me before and it jsut happened . I am on the pill and have taken the 3rd pill of the last week (week4) . But also last week I had sex on Monday and Wednesday. I am quite stress and feel as though I should be concerned.

My period was supposed to have happened around February 18 but my birth control threw off my cycle. I had some spotting dark/drown blood before Saturday. I start to bleed normally on Saturday and today. Although I normally bleed heavily but it’s now lighter so I’m wondering if that’s because of the birth control.