If I was in your place, I would do the following things.
- Drop in tomorrow morning and speak to the teacher before school starts (our school has an open door policy, if it didn't, I'd ring the office and make an appointment to see her). I wouldn't jump in with accusations, instead I'd say that DD seemed more injured than was at first apparent, and that her version of events was a bit different. Then I'd listen to what the teacher said.
2.If that discussion didn't satisfy me, I'd make an appointment to speak to the HT. Again, I wouldn't jump in with accusations, i'd just explain what had happened and listen to what the HT said. I'd expect the HT to say she'd talk to the teacher concerned, and come back to me.
For what it's worth, on what you've said, I think
a) Dodgeball is a very acceptable game for 8 year olds. Not played with a cricket ball, obviously, but a football or a sponge ball or a beanbag type ball. When your DD says it was 'hard', that is her perception, it may have been a basketball or similar, which is fine. So I'd get the facts about that.
b) They were obviously playing on grass, since she got grass in her mouth when she fell. That too seems fine.
c) I'd question myself about DD's pain threshold. I have one DC who takes everything very personally, so tends to see every bump or jostle or accident as intentionally directed against them. I'm well aware of that (from everyday life at home) so I don't jump to the conclusion that a teacher/fellow schoolchild is harassing them at the first complaint. That said, of course it doesn't rule out the possibility. I'd also be aware that if you are getting very worked up about this incident, it will encourage your DD to 'please you' by piling on extra details that may not be entirely accurate. That's how children work.