Major drop in FPS on TF2

Im running a late 2008 Mac Book pro with a 2.4 GHz Core 2 Duo and 4gb of ram. I can usually run tf2 and get 45 FPS in battle. The past 2 days I am lucky if I can get 15 in battle. I tried turning all of my settings to bare minimum and I was still getting 10 FPS. I ran Chris' max frame rate settings and same problem was happening. I tried L4D2 last night and I was running great a great frame rate the whole time. Any ideas what this could be?