IUD pinkish discharge??

I've had the Mirena IUD for almost 2 years. For the last couple days I've been having very light pink discharge and it went to very light tan. It's only happened to me once before and I wasted money on expensive pregnancy tests from late periods on the IUD. They always come back negative and. I never end up pregnant. But I just want to know why I get this pinkish tan discharge? Has anyone else gotten it before?