A pre-shave oil may give a slicer surface soling some of the harshness problem from the disposable blade.

I suspect the biggest problem is a face not used to a blade shave and a right handed barber who had less control of his pressure on the left side of your face. It is a fin line between perfectly smooth and burned painfully, especially with a disposable blade. This is actually a line I've crossed myself when I get a little agressive with a new DE blade in my blade holder.