than it's slipping. If the clutch was installed incorrectly and the pedal was not adjusted properly, you can burn it up pretty fast. It can also become contaminated with oil if the rear main is leaking, which you stated was the reason the transmission had to come out in the first place. A good mechanic would have inspected the clutch disk/pressure plate/flywheel and reinstalled everything according to spec IF everything checked out. If not, they should have called you to inform you that your clutch is toast.