Spotting mid-cycle with iud

Katie
I have a paragard (copper) iud in place, and have had it since soon after my daughter was born (almost 18 months since it was placed, now). I've never had the infamous heavy flow or bad cramping people generally complain about, nor have I ever spotted before, until recently. These past three cycles I've had breakthrough bleeding 8-10 days before my period is due (followed by a normal period last two months). This month it's been less like spotting and more like a light flow, and I'm not due for period for another 9 days! I know that I don't have pcos or any stds. I did do a pregnancy test the last two months, negative, so I'm not worried about that either. 
​In fact, I'm not worried so much as curious as to what the heck is causing this. I've never had mid-cycle spotting in my almost 15 years of periods. It's not exactly making me feel great.