Oh yes Trinity, I can relate to how you felt. The same thing happened to my RB Little Squirty-Bums. I can only guess how she must have injured herself, but her symptoms were the same as your Oatmeal's. We brought her to the vet and after x-rays and tests, the vet determined it was an injury, similar to what happens to a person who steps off a curb the wrong way and twists an ankle. It hurts like heck for a couple of hours but then the pain starts to subside.
So just like my Squirty-Bums, your Oatmeal is going to be fine.

Give your little one headbumpies and kisses for me and tell him not to scare mommy like that again.