I have never had one done but have read about them and heard they are umcomfortable and painful.... From what I get from it, the dr injects dye in the area of the potential problem disc, and if pain is moreso present for you they know they have the right spot, as well as I think they take pics too-xrays or similar? something like that, I know I don't have the whole jist of the test down pat, but DO know if I was getting one I'd have someone with me just incase to drive you home. Ask whether or not you're allowed to take your pain meRAB (if you're on them) as pain is part of the test to knowing that its the proper disc from what I get from it. I don't think AGONY but I know it's painful.
It is good you're getting the test to see what disc is the problem, if I was under other circumstances I would too get a discogram prior to surgery or anything invasive.
Good luck to you.