Hi there. Women have irregular bleeding and mid cycle bleeding for a few different reasons. Some have it due to fibroids and others have it due to hormonal imbalances such too much progesterone production. The best thing to do is to see your doctor.
If you had unprotected sex and risked pregnancy, it also could be a chemical pregnancy (very early miscarriage) but that would be an isolated incident.
good luck
are you alright now...are your still bleeding ???