I feel like this after a good work out. After running a few miles, or on the bike. Sometimes after taking a hot shower. As others have said, it could be many possible things.
Most likely in her case, she is just out of shape and needs to take it a bit easier in her exercising. When we exercise, exerting energy, burning up calories, body demanding more oxygen, etc etc, the body needs to adjust. If you are out of shape, the body has to work harder to do so. Proper diet will also help by giving the body the right nutrients and what not.
also, not sure what this all has to do with a bitter taste after sweet foods.. unless I missed something