Anyone else have this happen?! Normal?

So I've had the Mirena IUD in since July of 2014. Since the IUD was placed, my periods have stopped completely with the exception of very very rare extremely light spotting that I can only see on the toilet paper after wiping. Anyways, I've started to get normal monthly periods again for the past 3 months. I do still have Mirena. Just curious if this is a normal thing? And wondering what's caused the change from no periods at all to having regular monthly periods again. If anyone has had similar experience or has any insight, any and all comments are welcome!! Thank you ladies!!!! :) xx