Bleeding/Spotting Between Periods?

I've had my Mirena IUD for a year now, I've never had any issues. I had my period one and a half to two weeks ago, I'm due for my next period in 16 days. Tonight, I started bleeding in the middle of sex. No pain or discomfort, etc. Not sure why I started bleeding. I checked the strings, same place they've always been; right at my cervix. Was wondering if any of you had something similar happen; ever you just randomly get your period two weeks early with no symptoms?