Losing Belly Hair

The first thing I would do is take her to the vet and have them check her thyroid and do a total blood count. This can be caused by a few medical problems.
The next 2 things are stress and allergy. If it is stress still, you can try to make sure things stay on a routine basis at the house. Let he establish her routine and try not to change much. You can also start putting some Dr Bachs rescue remedy in her drinking water or directly into her mouth. It will releive a little of the stress. (you can find this at most health and nutrition stores).
For alleriges, honestly the easiest way is to have an allergy test done. Ask your vet about allergy testing through Bio medical services or Antech. They will just need to take some blood and send it to either lab. It will find any food, inhaled or flea allergies she may have.