my ds had a similar thing when he was about 16 weeks. DD(3 at the time) was trying to help and roll him back over (he started to roll over then so was always flipping! she'd seen us roll him back and decided to 'help') anyway he screamed in pain and I lifted him for a cuddle, but everytime I touched that arm he cried. So I took him to A&E. They agreed that he had a problem, and to go for an x-ray, before the x-ray another doctor came in to take a look as they don't like x-raying babies, and he sort of pulled it about a bit more vigorously but not too harshly IYSWIM, anyway he agreed there was a problem and we were sent for the x-ray. All this time ds had visably not been using the arm. Whilst waiting for the x-ray he seemed to be using it again which was odd.
Anyway we were called in for the x-ray and the man(can't remember what they're called) said that there wasn't much point x-raying him as bones aren't formed properly, they soft or somthing, and joints not developed fully(ds' issue was his elbow). I said that he now seemed to be moving the arm freely, to which he agreed and sent us back to A&E without an x-ray.
Anyway, my point is that they think he dislocated his elbow and the doctor being a bit more forceful clicked it back into place. Now i'm not suggesting you do that! I just wanted to share my story, although i'm notsure it helps you at all. But ds was absolutely fine straight after it clicked back into place,was using the arm freely again.
If you are still worried, take her back to be seen. They will avoid an x-ray at all costs on a young child. Maybe suggest you think it dislocated and see what they suggest? It is scary though. Don't wait take her back to the doctor or even go to A&E