So it sounds like you had it flipped to the single speed side?
It could be several things or one I am not thinking of.
1. Could be the chain is the wrong size
2. Sometimes when a freewheel is installed the mechanic doesn't tighten it all the way by hand since it will tighten itself when first ridden under a load. Did it make a grinding or metal on metal sound or just slip? If the latter is true and it only happened once it could just be that theand freewheel tigtened up and you don't have anything to worry about. I would ride tomorrow and just see what happens.