Hi there, I am reading a book atm that says that you can have mid-cycle or ovulatory spotting for a day or two around ovulation. Aparently it is considered 'normal' and it's a secondary fertility sign that can indicate a sudden drop of estrogen and tends to occur more often in long cycles. Considering you have had a + OPK I would say this is what it is. The author goes on to say that if there is fertile cervical fluid that is tinged with brown, pink or red colour it is considered extremely fertile.

Another explanation is that it could be an anovulatory cycle (no ovulation). I don't believe this would be the case for you considering you had a + OPK