The truuma which your daughter had because of fall must be very severe, that is why x-ray was required to rule out the fracture
in the arms.
As such the trauma just few days before may be itself a cause for the vomitting, but since you described that she had a hit her tailbone, so it can be related to that also. I would suggest you to keep an eye on the condition and look for any problem is there with the tailbone region.
I hope she must be taking some medication for the pain also, which may be one of the reason for the reported vomitting, but keep an eye on the condition and the pain in tailbone region.
Hope this help you. thanks