There are plenty of reasons your brakes might be rubbing, but one might be a piston that isn't retracting. As long as the piston itself isn't damaged, a little lubrication should get it moving again.
Keep some of the appropriate type of brake fluid handy, as well as a way to apply it to the pistons with some accuracy like a straw or a syringe. Remove your wheel and pads, and dab the fluid on the unevenly protruding piston. Push it back in and, while you're at it, expose the other piston(s) to do the same to them. On two-piston brakes, this is simple, but on 4-piston models you'll have to block off two of the pistons while you work on the other two. Gently press in on one piston with a screwdriver while even more gently pulling on the lever to expose the other piston. It won't take much. Cycle them in and out to get the fluid spread evenly.
Press the pistons back in, thoroughly wipe off the excess fluid, and put it all back together. Your pads will probably rest in a different position after you're done, so you'll need to re-adjust your caliper, but you should be problem-free after that.