What Causes Bleeding Before Periods Date?

My period usually starts around the 14th of each month. On the 7th I had something that looked like a period but it was very light and only lasted 4 days while my normal one lasts 7 days. Also i had no cramps, which i usually do. I am not sexually active and have never had sex. I don't know if this helps but i was on clindamycin for three weeks and just got off in the past two weeks and i looked online and it said it could cause hormone changes. What could have caused this? Thanks.