My cat starting dropping her rear end this morning and bumping into the walls. She also acts like she's hurting. What could be the problem?

I don't have an easy answer for you, the dropping her rear end sounds like a possible back injury. The bumping into walls sounds like something else, disease? Either way, if I were you, I would be at the Veterinarian right now. I wouldn't wait. Blessings...

How old is this gal? Indoor only? Does she act painful? Is she vocalizing? Only a vet can help you with this one. Things that come to mind: spinal infection or injury, abscessed fight wound somewhere on rear if outdoors, way out there... possible menengitis or other neurological problem.