Most miscarriages are actually unknown and happen within days of conception. Im not a doctor but thats my understanding. its possible you were pregnant. Start protecting yourself either way, your post tells me you wouldn't be ready for this =P
your definitely starting your period. that always happens to me after i go to the gym.
I am going to be as honest as I can don't be mad. Are you an idiot? Use a condom for f$%ksake! Please you don't want to get knocked up obviously as per your question, so please do yourself a favor use protection no matter what. Again just being honest, not trying to hurt your feelings but it needs to be said.