Help with car brakes?

This happens is when it's cold outside usually. When I put the brake on, instead of the car stopping smoothly it stops abruptly making the entire car jolt almost. I definitely jolt in my seat. It'll keep doing that until the car is fully warm. Even afterwards I hear the back brakes almost like they're dragging? I don't know if that's the word. I'll step on the brakes slowly and I'll hear the back brakes making this weird noise. The only way I can explain is you know when you're on a bike on a gravel road and you put the brakes on but the bike sometimes still skids on the gravel...that's what it sounds and feels like but the car stops no problem. It's making me paranoid. I've had all the oils and fluids changed and new brake pads about a year ago. What could this be and how do I explain to a mechanic?? I drive a 99 Chev Cavalier