What Causes Sudden Heavy Bleeding With Clots During Periods?

I am 31 yrs old.. a mother.. I am usually always normal when it comes to my menstral cycle..this time I am having heavy bleeding for about 5 days out of my 7 days cycle so far.. but htis morning I had a blood clot about the size of a half dollar maybe a little bigger.. and I am having a lot of heavy bleeding but no signs of dizzyness or weakness from the blood loss.. but I am now having heavy bleeding to the point of changing a pad every 2 to 3 hours.. I recently had knee surgery for the second time last month so is it because of me taking different medicatioins or something to do with that or should i be concerned..