Tmi? 13 DPO and I'm spotting. AF due Wednesday

I took a pregnancy test this morning and it was negative. I felt a slight stomach ache like constipation cramping (but I wasn't actually constipated) then went to the bathroom. When I wiped, I saw pink on the toilet paper.  I checked to make sure it was actually coming from my vagina and it was. Could this mean anything? AF not due until Wednesday... I've been on the pill since high school, so I haven't actually had a normal cycle since then. This is my first month off it and I'm TTC.