What Causes Spotting Before Periods Followed By Heavy And Painful Bleeding?

i have 27 day cycle period.. this month it was due on the 18, but i got it on the 13 with light red for only one time then it went to light brown did not even had to use a pad for it. it was to light. but know i got a regular period on the 18. and im still on it .. no heavy cramp or pain..im 38 years old.. whats wrong