15 day period was light around the 7th day got a tad bit heavier today. Mild cramping like my period cramps. Protected sex on the 15th last month why?

Lots of things. Always rule out pregnancy first. However, many things can affect the timing and regularity of one's cycle (e.g. Weight changes, diet, stress, etc..). Don't stress. Keep an eye on it for now. If the changes are persistent for more than 5-6 cycles, see your ob/gyn.