BLU CIVIC

Ok, here the problem. My brakes needed to be changed. the brake pas in the rear had started rubbing into the rotor. so i ordered brake pads for the front and rear and rotors for the back. changed the brake pads on the front (even though they didn't need to be changed), changed the passenger side rear rotor and brake pads and started to change the drivers side rear rotor and brakes when i found something odd. the inner brake pad had fallen off (don't know how) and the caliper piston was grinding into the rotor. on the brake pad there was like no brake, just the pad.the it's odd because there was nothing wrong with any of the other brake pads just this side. can anyone give me some advise on how this could have happened and wh4ere i can get another caliper piston.