Did you use any chemicals while cleaning your bike? It could have been something that got sprayed on the rotor and transferred to the pad.
I had the same problem with my front brakes (disk). I took the pads off and found there was no pad left, so I replaced them. I need to buy new rotors and pads (F&R) this year though for like new braking. If you have a service plan, take it to the LBS and have them take a look at it. It wouldn't hurt.