We recently moved into a house that was built about 20 years ago. I'm remodeling the master bathroom. There is a shower similar to the one in the picture. We have not used it yet, but I noticed that when I stand in it, the shower pan flexes near the center drain. It sinks maybe 1/8" - 1/4". Is this normal or something to be concerned about? It is on the top floor if it makes a difference.