Sounds like the caliper is sticking. It really needs taking off, cleaning, suitable lube applying to pins, piston seals etc, and carefully reassembling.
As a get you home, you could try getting a can of aerosol brake cleaner from Halfords or similar and spraying the rear brake caliper, pads etc liberally and trying to work it free.
If you ride far/fast like this the rear brake will get
extremely hot
I'd guess it's been working up to this for a while, so maybe you've been riding like it to a lesser extent. A short slow run might be OK, but perhaps stop after a short time to see how hot it's getting. WARNING do not burn your fingers, they can get extremely hot!!!!!!!!!!
It needs fixing properly.
This problem seems so common it's a pretty pathetic effort from Suzuki. Disc brakes on bikes are not a new invention and there are plenty around that don't sieze up at the drop of a hat. Shouldn't happen!
