2000 Malibu Tranny issues

After driving for 30 minutes and parking, the car started up fine, but wouldn't move in reverse. I got it rolling back, then I was able to get out of the parking spot and drive forward. I had to floor it to even move, but once I got going, I was able drive normal, although it lacked downshifting power. I got it home, made sure the fluid was good. I changed it a month ago, with the filter. After it cools down, I'll see how it is, but it doesn't look good. Any ideas?