I did have unprotected sex before my period in march as well, 4 days before it was due to be precise, but wasn't ovulating at that time if the ovulating calculator I have used is accurate
The intermenstrual bleeding can be due to hormonal imbalance. Other causes can be hormonal imbalance(can be due to PCOS),hypothyroidism,cigarette smoking or underlying Chlamydia infection while on birth control, ectopic pregnancy.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.