Transmission slipping

If there is not enough fluid in the transmission, that could cause slipping. Stop driving the car and get it back to the mechanic that did the work on it. He might have left something loose while working, causing the leak.

It is also possible that this is just a coincidence too. Perhaps a seal was getting old and finally started leaking when someone worked in the area and jostled it around a bit. It is more likely that something was left loose, in my opinion.