I actually thought AF was coming 10 days late (23dpo), and then it stopped and I got my BFP the day after. The doc told me that slight bleeding or spotting was quite common but if it happened again, to let them know. It did happen again a couple of times, but I thought it was too early for a scan anyway and if I was to miscarry there was nothing they could do.
I am now 28 weeks pg, so it ended happily for me, but I know otherwise what it could have meant. Do keep a close eye on it. You can test again tomorrow or Wednesday to be sure. If it's negative, it probably an early mc, otherwise known as chemical pregnancy.
If it's still positive, it may mean that the bleeding is due to something else, such as cervical erosion, which is not dangerous. Worst case scenario would be an ectopic, in which case, you need to find out and have it sorted out asap.
Fingers crossed for you!!