How can I tell if it's implantation bleeding or just a side effect of birth control pills?

I recently started on birth control pills (progestin only, if that makes a difference). It was about a month ago that I started. My periods are normally very regular, and my last one was. It started two weeks ago yesterday (September 24) and ended on that Friday night(Sept 28). But then yesterday, I noticed I was bleeding. It started out a brownish color but is now more red, but it's not very heavy. Does this sound like a side effect of the pill or could it be implantation bleeding?How can I tell if it's implantation bleeding or just a side effect of birth control pills?
It's probably the pill, but just to be sure, call your doctor. While combo pills prevent ovulation, progesterone-only (mini-pills) do not...they just make the whole reproductive cycle unpleasant for a pregnancy to occur (thin the lining, etc). So if you took your pills EXACTLY as you were supposed to and were NEVER late (not even by an hour -- timing on the mini is VERY important) then the chance of pregnancy would be less than 2%. But again, check with your doc.