Open up Activity Monitor (in the Utilities Folder) and make sure it shows "All Processes" at the top right.
At the bottom you'll see the tabs. If you click on CPU you'll see if there's a runaway process that's pegging the CPU. Report back if something bizarre shows up.
Click on the System Memory tab: if you've got a lot Page Outs, do a restart. The restart in itself may cure any overheating.
PS: what kind of temps are you seeing?