Does it feel as if there is no suspension travel at all? Like you are riding on no suspension?
I had this happen to me when I replaced one of my struts. It was because I didn't pressurize the struts when I put them in. The fix? Rear end gets jacked off the ground, the compressor needs to be engaged to pressurize the struts , pull the jack out so that the car has the rear hanging in the air from the elc, then allow it to settle back down. I bounced mine while it was coming back down, and it settled perfect. The rear end was still a little high, but it rode fine afterwards. It is back to normal now.