I suggest that the breathing difficulty is caused by anxiety. I can think of no medical reason for the stomach and breathing symptoms to be related unless the stomach pain is also aanxiety related. You could check with the advise nurse.
I suggest that when she has trouble breathing its essential you remain calm while you reassure her she is ok. Have her take slow deep breaths while you count them. She will feel OK in a few minutes.
The difficulty breathing is not uncommon. I've seen it several times and experienced it myself once. She is OK. Keep track as the doctor asked. This will help to find a diagnosis. In the meantime try to be less anxious yourself. Tell her you're concerned, that the doctor will find out how to help her and in the meantime she's OK even tho she's uncomfortable. This is not life threatening.
I just read your previous post. Good to see specialists and have tests. Know that when they've all been done you may still not know why she's having pain. You will know that the pain is not caused by any illness known to these doctors and it's extremely unlikely that her life is in danger. Please consider the possibility that she was having some pain that is now made worse by her anxiety. Anxiety causes chemical changes in our body. Some bodies are more susceptible to this than others. Physical tension can cause tge body and joint aches you describe.
I know this from personal experience. When I was working I ached most of the time. My joints hurt. I had medical tests which found no cause. I also had very painful irregular periods. My primary care doctor told me that just because they coukd find nothing wrong didn't mean nothing was wrong. He said he would help me be more ccomfortable by treating the symptoms. Medicines did help.
Many years later the medical world became aware of endometriosis which turned out to be the cause for my menstrual difficulties. But I still had the body aches and joint pain. Twenty years ago I retired from a stressful job. Within a year the pain decreased. I have been free of joint pain and general body aches for many years now. I credit this to both less stress and recognising and treating my anxiety and depression.
I'm not saying your daughter is depressed but she certainly is anxious. She is sick and doesn't know why. Anyone would be scared by that. I am suggesting that her anxiety could be making the pain worse. It's very likely her breathing difficulty is caused by her reaction to her intense pain.
I urge you to give her the medication prescibed by the doctor. Unless he said to only give when she's in pain I would give it on a schedule. Sounds like the doctor suggested it because it has the potential to help. So give it a try. Doing so is part of the diagnostic process.
I am suggesting that she may have a physical cause for her symptoms but to try to be OK with whatever is found or not found. In the meantime treat the symptoms.