Pain from root c?

I had a root c in my back top tooth, and then a cap put on it about a year or two ago. Now, there is a black line showing from underneath the cap, presumably because the tooth is dead underneath. But I am feeling HORRIBLE pain from this tooth all of the sudden and I have no idea why. I am going to see my dentist tomorrow but I was wonderin if anyone knows a way to help the pain, besides normal pain killers. I haven't slept because it hurts so bad, and I usually have a high tolerance for pain. Is the tooth going to have to get pulled?