What Causes Repeated Menstrual Cycle?

I had started my period on 3/8/15 and it only lasted 3 days. I was on antibiotics at the time and I m not on birth control. That Wednesday I began to have clear discharge, and it continued until Saturday. It was completely clear and thin and almost the amount of a regular period. Today, 3/17/15, I noticed light bleeding. this hasn t happened before and I don t know what caused it.