I've got this really bad taste in my mouth - kind of on the back of my tounge and the roof of my mouth, right at the back. I don't quite know how to describe it - bitter I guess - like if you try to take an uncoated paracetamol and it sits in your mouth too long. It's been there since yesterday morning - I've brushed my teeth, had things to eat but it keeps coming back. Does anyone have any idea what this could be?