Should my retainer hurt even if I wear it every day?

Two days ago I got my braces off and my orthodontist gave me a clear retainer for the top and bottom of my teeth. He said I should wear them every night. I know if you don't wear your retainer your teeth shift and when you try to put it in your mouth it hurts like hell. I however, have worn my retainer for the past two nights and it's agony. Is that normal? Also my bottom retainer doesn't hurt too much, it's mainly the top one?