We're you actually purposefully hit in the stomach or knocked when the kid shoved past you or something? There is a big difference. If it was the former is be demanding a crisis meeting immediately. If it was the latter I would be setting up a meeting with the Principal & your union rep to outline your concerns and requests.
Your Principal may be incredibly supportive if you give him/her a chance to be so.
As for the 2hrs in the hot sun, if it was a sports carnival or something I don't see this as unreasonable. PE teachers still do their jobs when pregnant, just make sure you have a hat and plenty of water. Sit in the shade when you need to.
I would be telling the class your situation also. Often a great way to win over difficult students is to put your trust in them - tell them you are pregnant and need their help to ensure the baby is safe. Explain that they need to be extra careful around you. In my teaching experience boys can be incredibly gentle and kind.
Do you have a mentor or colleague you could ask advice from for some behaviour management strategies?