IUD running out of hormones?

I have had my mirena IUD for 4 years now. I have the 5 year one. Anyways, I used to have really bad periods, heavy bleeding, bad cramps etc. with my IUD, all of that completely went away. The past few months I've been getting a period again, with lots of bleeding, and perfectly on schedule. Could the hormones in the IUD be running out early? Or did my body use them up too quickly?