Perhaps the caliper is sticking. That could be an internal problem with the caliper piston and seals or it could be (more likely) the caliper slide pins.
A caliper that is stuck can, in extreme cases, feel like a warped rotor. Did you lubricate the caliper slide pins when you installed the new pads? Proper lubrication of anything that slides and any part of the pad backing plate that contacts the caliper is one of the most neglected but important items of a good brake job. It makes the difference between a mediocre and a great break job.
