Pleiades, is spot on as it will be either the pistons sticking (less likely as some pistons are moving hence the heat but you might have one or more stuck) or the caliper slides seizing. Either way, you really need the caliper torn down and given a good clean, grease (slides) and rekit if required or simply reassembled if the seals and boots are OK.

If you're not that confident, although the job isn't hard, its not something you should stumble along just in case. Do you have someone close that could give you a hand for an hour or so?

BTW, it doesn't take much to semi-seize a piston and you will push them out due to the hydraulic advantage you get on application but it is only the piston seal that retracts the piston when you let go of the lever, hence it applies poorly and drags on release. Its a square section seal (as opposed to an oring) and as the pressure builds up behind the piston it moves forward and the seal distorts. When you let go, the distorted seal is what pulls the piston back to a released position. This is also why disc brakes stay adjusted...if the pads wear, the piston has to move out further to grip the disc and consequently has to move through the seal to a new adjusted position.



Additionally, this action is what makes a warped disc a dangerous situation. While riding and flexing the wheel the warped disk will push the piston back into the seal, so on first application of the brake...it will take alot of lever movement to apply the brake and increase application time. In a bad case it may take two pumps of the pedal or lever to actually grip the disc.