Homesick Rae,
if there is no biological cause, it could be a stress symptom.
It can disappear for while and then be back suddenly when you does not expect.
It belongs to the whole psychosomatics symptoms of stress.
It's a warning sent by the body to tell you that it is enduring something it can't stand anymore.
Very often, it's not the "someting" that is responsible but the way you handle it.
Hope it will help.