OK so i just had this exact same problem on my 240sx.

1st a little back story. I had just installed a new Brake Master Cylinder and my brake lights were staying on. I found that I was missing the little rubber button on the brake pedal that hits the brake light switch when released. I adjusted the pedal and sensor as much as i could to attempt to turn off the brake lights anyways. This did not work. Finally I was able to find the part number online and got the rubber button thingy from my local dealership. Right before heading out on a road trip I popped in the little button. Now after a few miles on the highway my brakes started feeling very strange, and my car felt weak. The car kept going slower and slower and the pedal kept getting firmer and firmer. Eventually I pulled over and my front brakes were smoking like crazy! Finally figured out that the previous brake pedal adjustment plus the added brake switch rubber button was slightly pressing down the brake pedal and slightly dragging the brakes. I guess as the brakes got hotter, the fluid (ATE Super Blue) expanded instead of boiled, which added more brake drag, which caused more heat and started and endless cycle until the brakes were completely seized.

Slim chance this is what your problem is unless you have recently messed with any of the components of the brake system or adjusted the pedal or brake light switch.