Any ideas why my shower pressure suddenly dropped?
Something could have broken in the shower valve.  Most serviceable parts can be removed without having access to the wall.  Each valve is different, but the normal steps are to remove the handle from the valve and then unscrew the valve core, or remove a clip and pull it out, or unscrew a collar, etc.  You'll have to turn off the water to the house to work on this valve because there's no way to turn off the water just to the shower.

I've had this exact scenario when a damaged rubber washer tore in two and clogged the internal workings of the valve.