What Causes Light Bleeding, Cramping And A Change In Menstrual Cycle?

my periods have been a 24 day cycle, last month it came late and when i caluclated it now turned to a 28 day cycle. i was expecting my next period on the 3rd of december but on the 29th of november i got a bright red blood and so crampy. its very light. what could be the problem. and it will help to know that i too some vaginal tablet for yeast infection.