What is causing my toothache?

I started getting a toothache yesterday afternoon, and it hasn't gone away. It's a throbbing pain and it gets worse if I eat sweet, acidic, hot or cold foods. I'm not sure if it's decayed because it's at the back and at the top so it's impossible to see in a mirror. I don't know whether it's decay, acid erosion (my dentist told me that I drink too much fruit juice), exposed dentine (I grind my teeth a lot without noticing) or any other cause. I can't go to my emergency dental clinic because I'm in Canada for the next three weeks and I live in the UK.