You overheated your clutch. Autoclutch isn't very good at slipping the clutch, so it's quite easy to overheat it if you don't drive taking that into account.
O.k. thanks guys, I was keeping my foot in it during the shifts.. I'd get a little out of shape in a corner, then bang through the gears trying to gain back lost time. Guess I wont gain anything if I burn out the clutch.
You can check the clutch temp in the F9 menu as well, its the bar labelled CT down at the bottom. If it gets to the red, then you're doing something wrong.