I had something like this also. I was worried so I went to see my OB. Turns out that its apparently normal and more common than I thought. I wouldn't worry about it too much, unless it gets worse. I had the same issue and it only lasted a few days. I did notice though that i had a lot more discharge when I had my IUD in. Like you, I had no periods which was kind of nice.
The hormones an IUD puts out with change up your mucus and pH in your reproductive system a lot. It can cause nasty discharge, irregular spotting (which could be what's happening to you) and cramping. Sometimes when spotting the blood with be fresh inside, but by the time it comes out, it's old and brown. I would still talk to your GYN and make sure everything is working correctly. Also, if you've been having intercourse, it could have been knocked about. Rare, but possible. Good luck, and talk to your GYN.