Definitely talk to your physician, ask for an MRI, this is the best way to find out what is going on. X-rays are good, but they do not always show the entire picture.
IF and only if, all tests come back showing nothing serious and the doctors have ruled out other issues, then maybe a visit to a chiropractor could be visited, one that specializes in geriatrics, if not a chiropractor, then a good masseuse or physio-therapist.
Make sure you see the doctor first however.
You can ask the doctor to refer your girlfriend for an xray.
What happens the doctor will write a note for the hospital radiologist, saying which part of the body needs xraying.
You do not say how old your girlfriend is, but as your profile says that you are 78, I presume your girlfriend is a mature lady and if that is the case, it is highly likely that she has degeneration in the thoracic area of her spine.
The doctor is the best person to speak to and will be able to give your girlfriend advice. It would also be useful to record any other symptoms like pins and needles anywhere.
Best wishes.
Are there any other tests(like a chest X-ray) she can take that could either diagnose or eliminate the source of her problem?
It sounds very much like she may have a problem with the thoracic area of her spine where there may be some nerve impingement.
If she notices that this happens when she does anything in particular to set this off, then she should avoid doing whatever is triggering this.
The doctor would be able to refer her for xrays or other imagining to find if there is any problem in the spine.