Pick up a water pressure gauge at one of the big box stores or hardware store for about $10 and check what your house pressure is. You can screw it onto the drain of the WH, or washing machine, or outside hose bib. If you pick up the appropriate adapters, you could even screw it onto the showerhead arm.
If you have a pressure reduction valve, it may be shot. If you have an expansion tank (and you should if you have a PRV), then it may be shot. Then, when the water heater turns on after use, the cold water expands and can cause the pressure to jump up way over 100# until something gives (usually the T&P valve at the WH).