What is the cause and how do I prevent these stomach problems?

For the last two years I have gone to a six day (Sunday to Friday) hockey camp. The first day I am fine, but on Monday my stomach starts to really hurt, it feels like there is a giant rock in there and I have explosive diarrhea (sorry to gross you out.) It is really embarrassing to use the restroom, since it is almost never private (meaning there is only stalls to use where other people can hear.) Last year, on Wednesday I found Ginger Ale in the the cafeteria and it didn't hurt after that. So is it homesickness, is it the food, or is it something else? I am going to a different camp (it's the same length) this year, so if it is the food, hopefully it will fix it. PLEASE HELP! How do I stop this?!? It makes the camps not as enjoyable as it should be. I was thinking about bringing Tums this year, maybe that would help? If I do bring Tums, when should I have them because we don't get to go to our rooms anytime besides to sleep and change? Thank you!