Could be wrong, but I think they're probably checking for a missed chest infection/pneumonia.
You can get atypical presentation, this has happened to ds2 twice and also to me as a child.
The first time ds2 complained of leg and stomach pain - no coughing - turned out he had severe pneumonia that, if we hadn't caught it when we did, would have encapsulated, making it even more serious. The stomach pain was from the swollen lung pressing on his stomach, the leg pain was referred. Second time, no pain or coughing, just vomiting every night when he lay down.
With me I never had a cough and was ill for 6 months before I collapsed and after lots of xrays from different angles they discovered I had life-threatening pneumonia.
The good news is that in all cases it was easily identifiable by xray and we were both fine after the right anti-biotics.
If they're not sure what it is, they're probably just trying to rule things out.
Only other thought I had about right side pain is the gallbladder, unusual but not unheard of in children. Not sure if that could be seen on an xray though - they usually do an ultrasound to check it.
Hope they get to the bottom of it and she's feeling better soon.