I've had spotting between periods a few times; this last time, I was given a progesterone test n cd21 and found to have low progesterone, so I was put on supplements. Your progesterone has to be at a certain level to maintain a healthy pregnancy. Has your Progesterone been checked? The other times I had bleeding I was found to have polyps, which had to be removed through D&C.
I went to my Dr. after 4 cycles telling him that I start spotting 7-8dpo until AF arrives and he had about ZERO concern. As long as my cycles were regular, OPK's were turning POS he felt I was ovulating and had no problems. I got pregnant that month..Even with all the spotting, good luck!