Number one problem. You can't use stock shocks with lowering springs.(assuming you have lowering springs, if not please ignore) That's a no no!!

Do you have a rear sway bar? It maybe out of adjustment.

Is it sitting on level ground? I notice the same thing with mine from time to time when it's not sitting level. I know this is a stupid question, but it was worth a shot.

Has the car ever been in an accident?
Do you have anything heavy in your trunk like a sub, or amp?

What year and model car is it?