You'll be alright from him sitting on you, if for some weird reason you were going to be sick from it, it would have happened immediately. besides I remember being a kid and one of my brothers jerky friends kicked me HARD in the stomach and I didn't get sick and that was as a kid so you should be fine. Also as mentioned your period can make you feel blah and being the first day back nerves can make you feel a little funny. Once you get back into your normal school routine again you should start feeling better and by the end of the day you will wonder what you were so worried about.
\"As soon as you trust yourself,you will know how to live.\"
Johann Wolfgang von Goethe
\"Do not anticipate trouble, or worry about what may never happen. Keep in the sunlight.\"
Benjamin Franklin