What kind of treatment should I seek for intense nerve pain along my butt and hamstring?

Last night I noticed that my lower back and butt were kind of hurting. This morning I woke up and went to my personal trainer and on the very first exercise, a squat, I had EXCRUCIATING PAIN shooting down my left butt cheek and all the way down to my feet. It was the worst pain I think I have ever felt. I had to not even put my foot on the ground because it hurt so bad. I am 23, fairly active, fit, and healthy. I have no idea what this is, or what is causing it and I need advice of what to do to get better! PLEASE HELP ME!