What Causes Heavy Bleeding And Cramping While On Chlomid And Endometrine Medication?

i am actively trying to get pregnant and have just started some heavy bleeding and cramping today on day 21 of my cycle. My last perioed was on March 11 and is usually around 26-28 days. I have been on chlomid and endometrine the last few months but was not on those meds this cycle. Wondering what is going on?