My period had stopped and i had clean tampons for a day. but today, two days later, it started again with heavy bleeding and lots of clots.

Can be normal: Menstruation sometimes does not act exactly as we would want it. So sometimes periods can start and stop then start again. A period is the shedding of the lining of the uterus and sometimes it is not complete. I would be concerned if the period continues for more than a week (>7 days), or if there is increasing pain/fever. If there is you should go to your physician for a check up.