I began to have stomach problems, and my body coud not digest meat at all. So I researched and studied about what vegetarianism was and how to stay healthy. I think maybe my mom liked that I did research, so she started to allow me to not eat meat.
I also had stomach problems a few years back when I breifly started eating meat again, so I completely sympathize with you on that. Digesting meat is rough!
I didn't have stomach problems when I ate meat, but I had headache very often and it stopped when I started my vegetarian+fish-diet.
But two years ago or so I started to have problems with my stomach after eating dairy. Not always, but sometimes. And lactose free products aren't any better than normal.