For a period of time my the hot water in my master bathroom shower won't
heat up to the usual temp. All other faucets in the house, including the
bathtub (separate) next to the shower, and two other showers, all heated up
properly. But just as I was about to call in a plumber it fixed itself.
E.g. the water gets hot again.

I am thinking this has to do with a possible blockage, which somehow
cleared. Should I still call in a plumber to look at it? This is probably
going to be a major job, involving breaking through the walls.