Sarah!
Glad to have you with us!
I'd stay away from protein bars. You can look at the ingredients and try to find one that doesn't have any trans fats (nothing that says hydrogenated), only has whole grains, and is sugar free (including fruit juices, honey, etc.). It's hard to find, but there are some out there. The thing is, those bars don't give you much in the way of protein or veggies...they're mostly carbs, which you burn quickly. I'd go for a protein/veggie combo instead, like 3 bean salad, bell pepers with hummus, etc. when working out, or even a protein shake with protein powder, yogurt, etc. that you make at home.
Was your stomach acidic before? I'd go see your doctor on that one just to make sure you don't have a physical condition. If you are having that problem all the time, it might be something more serious. If it happens once in a while, you can try antacids, but look carefully, almost all of them are fully of sugar. I'd stay away from them in P1 because I've never found any sugar-free ones, just ones that are low in sugar.
Feel free to post any other questions you might have. This is a great community with really helpful, knowledgeable people!
First Time at the Beach
to the Beach. Are you eating dairy? Milk may help. Remember that there are new foods added to the Phase I list - check out our FAQ.
howdy.. hope you enjoy the beach!
We all (well most all 
) stray from the beach fer awhile... and yup fiber can give you such stomach cramps... Review the stickies for some great information...the recipes are all wonderful and have become regulars in our meal rotations... and visit the daily thread to say hi , lend and give support and advice....