What can cause Dysgeusia (Parageusia) in the mouth and with smells?

My girlfriend is experiencing what seems to be Dysgeusia (Parageusia). She is smelling and tasting a constant burning plastic or rubber. What could be the cause of that? She brushes her teeth and doesn't take any pills except for her pms pills and she hasn't started yet. What could cause this? Extensive insight would be mucho appreciated.