Hello!
I would also suggest you to get her screened near a cardiologist and a chest specialist to make sure that everything is fine with the circulation and respiration.
After this you can concentrate on other physical things which lost_myself has pointed out.
Counsel her and teach her some breathing exercises. Encourage her in whatever sports she is comfortable. Avoid all forms of exercises which are giving rise to these symptoms. Check her weight and see that she is in proper BMI.
If nothing works out then Psychiatrist is the best person to diagnose any anxiety related issues or panic attacks.
Take care!
It could def be a medical or physical issue and you should make sure she is screened well. However since you mention that it now happens at any time, it sounds very similar to a mild panic attack, it could be the onset of a panic related issue, GAD or something similar. Evaluate her outside stressors , 16 is a tough age for some. Also ( not insinuating your daughter is bad) kids this age age tend to experiment with drugs and alcohol these , especially marijuana and alcohol, can make anxiety syptoms worse for some, even though you'd think they would have an opposite effect.
Look in the anxiety forum here to get more info and talk to your daughter a bit and her doc.Good luck.