The Polycystic Ovary Syndrome (PCOS) possibility is ruled out here. This might not be an implantation bleeding, as even those having cycle duration as low as 24 days. You can also rule out the pregnancy chances in this case; ovulation in this case will occur at day 10 and it will need another 6-9 days for egg implantation. The bleeding between periods can be caused due to many reasons, for example, hormonal fluctuations, low thyroid levels, stress, a vaginal injury, vaginal infections, or spotting duration ovulation.
The major cause for between period bleeding is hormonal fluctuations found during ovulation. Ovulation occurs when estrogen and luteinizing hormone interact. In order to make the interaction possible, there’s an abrupt increase needed in the luteinizing hormone along with a steady increase in estrogen. This abrupt hormonal change makes the sensitive endometrium of the uterus to partially fall off, which results in bleeding. Also, the wall of ovary also has to break for the follicular contents to pour out. This also results in vaginal bleeding.
The possibility of a vaginal injury or infection cannot be ruled out. If the bleeding lasts more than 48 hours, it might not be ovulation bleeding. Any other symptoms like itching or burning can place it in a vaginal infection category. An appointment with a GP/OBG is necessary to clear out on the issue and get the needed help, if any. Also, if you don’t want kids at the moment, it would be better if you use some kind of protection during sex; it’ll keep you from worrying every month.