C.J.
Where is the decoder ring for children when you need it?:) The first thing that popped into my head is did you all do something new or different that would have caused her to use her glutes more than normal? Swimming, jumping up and down (excessively), a sprited play date, trip with a lot of stairs. I know, silly, but it sounds like a muscle issue since no tummy grabbing. I know that after a change in my routine it can cause me discomfort. My boys always complain of sore muscles after a day at the water park, family vacation where we hike, etc.
You could try an appropriate dose of children's advil or a topical muscle cream to rule that out before you call the DR.
Hope she starts feeling better soon!