Why do my back tires spin when I hit the brakes?

Today when parking I pulled in the spot, and stopped, but my back wheels spun! After they stopped I put it in park and got out. After maybe 3 minutes the car suddenly lurched forward and rocked for a few moments. It did not actually leave the place I parked it, just kind of jerked forward then rocked, slowly resettling. What is wrong with my car? I have kids, I can not have a crazy car :(