Check the brake pins. If the pads are hanging up on the pins, they may drag against the rotor and create heat. They may drag because they need lubricant, or because there is some wear or corrosion on the pins that is keeping the pads from moving freely. They make a special high temp grease for brake pad pins; you can get it at most any auto parts store.
Clean the brake piston. If the piston can't retract fully, it may be putting slight pressure on the brake pads, causing them to drag. Take the caliper off, put a small 1/4" piece of plywood between the pads, and squeeze the brake. This will extend the piston, and the plywood keeps the piston from extending too far. You can do it without the plywood, but just keep a close eye on the piston as you extend it. Once it's extended, spray it thoroughly with brake cleaner. Take a piece of nylon rope (you don't want it to shed fibers), soak it in brake cleaner, and use the rope wrapped around the piston to scrub it and remove any stubborn grime. Push the piston back into the caliper.
I'd also bleed the brakes, to be sure.
There's always a chance the rotor is warped, which might cause brake drag, but it'd likely you'd notice that when you were braking while riding.
"No matter where you go, there you are."