Sounds like critical incident stress.
Something unusual happened and your body switched on the fight or flight system because it was so unexpected (and painful). The unfortunate thing is that the system doesn't have an immediate off switch. The episode is over and there you are still juiced up on adrenalin with nowhere to go and nothing to fight. Add one little shit of a teenager and to you the sky is falling (I'm sympathizing not criticizing).

You are actually doing what's best to do. Talk with others.
It's definitely not your fault it happened, although if he is autistic I can't say that he is at fault either. An Autistics mind does not work the way ours does. Unless the injury aggrivates the old injury I don't see suing anyone. The child does need an assesment to see if he/she should be placed in a special needs class. This is for your protection and the protection of the other students. The parents may bitch but safety is paramount (especially if he acts out against a larger child who might injur him fighting back.) Forget the teenager. taking some time for yourself might be good. It might also take a few weeks for the body to settle back to normal. Realizing that it is a critical stress incident will help you deal with it.

I had two episodes a week or so ago that left me in the same condition. First one of our friends a the bowling alley who is handicapped had a seizure. I helped him the best I could (not a hero, just in the right place top do the right thing). The next evening we had a suicide threat which I had to deal with. Instead of feeling fantastic about my actions and great about both of the postitive outcomes of both situations I felt worse than ever. You'll feel better in time just like I have.