Brakes locked up three times at slow speed?

I went about a mile from home and then back home, because three times, as I was pulling away from a stop, as I got up to 5-10 MPH, it felt and sounded like (the tires even screeched slightly). I had stomped on the brakes, but of course I wasn't touching the brake pedal. The first time was as I pulled out of a parking spot in front of my house, where it had been parked for 36 hours. After the first time, I began to go back into the parking spot i'd been pulling out of and they felt as if they'd let loose, so I tried going forward again and everything seemed fine. It didn't happen at every stop sign or stoplight, but each time it did, it seemed to let loose if I went backwards a couple of feet and then forward again. After the third time, I headed back home instead of continuing on my errand run. As I got to around 30 mph, there was a kind of loud clunk noise, in the driver's side rear. That was followed by an intermittent noise that was like a brake pad rubbing on a rotor. Of course, because I was trying to get it home and there weren't stop signs or lights on that stretch of road, I hadn't hit (or touched) the brake pedal at all, just prior to the loud clunk. That noise, hadn't seemed to even slow the car, nor had it locked up the brakes.