I have a stand up shower in the master bath room which has 2 handles (hot and cold). I do have well water and the system is about 20 years old. One day I was taking a shower and lost suddenly lost lost pressure to where it was only trickling out. I had an old shower head so I replaced it thinking it was done. The new shower head worked perfect for about 10 minutes and the same thing happened again where it was working and then suddenly down to a trickle. Unfortunately this shower does not have any shutoff valves so I'm not sure what the next step is that I should take?

Check the pressure tank. The small pipe going from line to tank may be clogged.clean pipe.

Check air pressure in tank. Should be 2PSI lower then turn on PSI. This check is done with tank empty.

Chuck