I Am On Letrozole 7.5 Mil From Day 3 To

I am on letrozole 7.5 mil from day 3 to 7 of period and I have been getting regular periods. I have pcos. This cycle my period is 6 days late. I have no pregnancy signs other than I am getting hot all the time. I have a two year boy who I did conceive on letrozole. I really don't think I am pregnant but could letrozole suddenly stop working and my periods are interrupted?