Is this day over yet?

So yesterday I ran 4.5 miles around my complex. No problem. Kept at about a 12:00 mile, which is my estimated pace for the race. I ran on the treadmill tonight - just for 30 minutes, but I increased my speed slowly to faster than I've done before. (That's what was on the training schedule for today) No problem. (And when I say no problem, please translate that as "I didn't die") Then I got off the treadmill and started walking across the gym - I was headed upstairs to do some weights. Suddenly EXCRUCIATING pain hits my knee. Out of the blue. I didn't cry (inside the gym), but it was tough. I couldn't bend my knee and I could barely, barely walk. I slowly, slowly, slowly hobbled out through the sleet to my car. By the time I got home, the pain had lessened, but not gone away. Although my knee has been giving me some problems, this kind of pain has only happened once before. I've bought a brace and I'm wearing it now. I think I'm going to have to go see someone about this knee issue, but I have no idea who to go see about it. I think I'll ask a trainer either at the gym or at my training run on Sat.