NEED HELP! Clunking Noise Scares me!
 

So I am driving on the freeway today on my way to a meeting. The day is already starting out bad cause my Fiancé was in a small accident, a lady ran a red light. But she is ok and the car is a bit banged up but she is fine.

I am driving 75mph (following traffic) and all of a sudden a big CLUNK appears from the right rear. I thought I hit something but it keeps coming back. I pull over to see if anything came loose, nope nothing, wheel is tight, muffler is on securely. I recently swapped out both which is why I checked. Now this noise is coming and going, it almost feels like the car is breaking but I am not 100% sure about that. My thought was the suspension mounting points were loose but can't check the rear myself. The noise keeps coming back and it makes a big clunk and feels as if something was pounding underneath the car.. Felt it in the center console area as well.

Anyone have ANY thoughts regarding this??