Celiac is a big jump from some abdominal pain, as others have suggested. If she's constipated, you DON'T want the BRAT diet! That's for diarrhea and nausea, and it's great after the stomach bug. But if there's some constipation involved and she's not going every day, you need to switch her up to more fiber. That means brown rice, not white. I'd cut back on the fried stuff - like French fries and even a chicken nugget. (You could substitute oven-friend potatoes with a little olive oil, and make your own nuggets with chunks of chicken tenders dipped in whole wheat bread crumbs or wheat germ, quick fried in some olive oil just to crisp up the coating and then finish by baking in the oven.) I'm not sure the pancake alone caused the pain - probably just the accumulation of grain and oil and dairy. I notice there were no fruits or vegetables in the day's menu - that absence of fiber is probably a big problem. Can you switch her off the fruit loops and onto whole grain cheerios, even with a little stevia for sweetness?
If you're concerned about the gluten in the pancake, you have to be just as concerned about the gluten in the cereal and the nugget coating. It's just as likely that the dairy caused the problem.
But the absence of fiber and (I'm guessing) not a lot of water is a much bigger problem. She may well have a whole ton of sludge in her intestine and it just gets worse during the day, perhaps more so at night when she's not as active. If she is not pooping once a day, she is fiber-deprived.
Before you go do a whole lot of invasive tests, why not make some simple changes by going to "darker" version of basic foods (whole wheat flour in the pancake with added wheat germ - she'll never taste it!), whole grain cereal with some fruit (anything but bananas which are on the brat diet and therefore the opposite of what you want), healthy chicken nuggets (I can give you a detailed and very simple recipe of what my son LOVES), sweet potato "fries" instead of white potatoes (or at least oven-fry them instead of deep frying), brown rice with a Chinese veggie & chicken stir fry, and even fiber-rich hamburgers on whole grain rolls. And water water water. Try some apple slices dipped in or spread with peanut butter or almond butter, veggie slices dipped in hummus, and that sort of thing.