I fell. Slipped on wet asphalt in a parking lot. Landed tilted on my back side, hip and butt. I think I was around 29 weeks...
It happened on a weekday, so it was convenient for me to call my OB and tell the nurse that I fell. They suggested that I come in; hooked me up to a fetal monitor, just to make sure there were no contractions happening, that the baby's heart beat was good, etc. (Everything was fine.)
I remember the nurse saying that, as long as you don't fall in such a way that you land on the baby's head (which I don't think you need to worry about, because at 33 weeks, he should be starting to face downward, space will start to get tight in there), then the baby is pretty well cushioned in a big water bubble. Pretty much, the belly is the safest place for him to be. (My sister was in a minor car accident while preggers and was told the same thing... and her pregnancy continued fine too.)
I do recall being told to put my feet up and relax for the first few days after my fall. It was not the time to start any "clean the house" nesting projects!
For piece of mind, you might want to call your OB tomorrow morning and tell the nurse that you fell. They might want you to come in for a fetal monitor. However if you tell them that you've had consistent movement, no spotting, no contractions, etc., then they might tell you not to worry about coming in either. But I would still call.
Ice those knees! 15-20 minutes, 3-4 times a day. Don't be surprised if your back and wrist/arm hurts a bit tomorrow too (depending on what parts of your body absorbed the impact). You can take some tylonal for the pain, however when I did my fall, I chose not to take tylonal because I wanted to be in touch which any possible contractions that might happen. Try to relax and get some good sleep tonight.
And a couple good spoonfuls of icecream always helped!