Before i see my period for five days and its heavy but now i see it for three days nd its not heavy again. what is the cause?

Many possibilities: Menstrual irregularities are extremely common and seldom indicative of a serious problem. If this becomes a steady and continuous problem your gynecologist can examine you to be certain there is nothing seriously wrong, but would suggest that this is probably quite normal and will correct itself. Good luck.

Normal variation : You wrote your periods normally are 5 days and heavy and now are 3 days and lighter. Most likely this is normal variation especially if the period is coming at the correct time. If you have any pregnancy symptoms like nausea or breast tenderness or miss a period check a pregnancy test.