Why am I bleeding?

My periods have never been regular by any means until about last July when I switched to Lo Loestrin birth control pill. My last period though, came 3 days early. It was pretty heavy/clotty for 2 days then tapered off to about nothing by day 3-4. Weird. My periods are usually heavy for 4-5 days then normal the last days (5-6).

So I thought it was odd but everyone told me not to freak out so I didn’t.

That was two weeks ago. Sunday I began bleeding again. Not badly- just spotting but it was brown. By Monday it was heavy and super clotty. And by Monday night it was gone again.

I dont know why I’m bleeding. I’m in my fertile window and haven’t had sex. My fertile window started on Friday and the last time we had sex was Wednesday.

Help?