My USB plug is getting physically hot?

I own a Microsoft Wireless Mobile Mouse 4000, and just today it stopped working. When I went to unplug the little dongle, i realized it was very hot to the touch. I got it out with some pliers, and upon plugging it back in, was told by my computer that the device was no longer functioning. So it doesn't work, and every time i plug it in, it gets very hot. Does anyone have any idea what is happening?