We've experienced the problem also, and when it happened to us, I attributed it to lime and such coming loose from the inside of the water heater and partially plugging the small tapered inlet in the shower valve. It's a real pain to remove, disassemble, and clean the valve, if even practical, I don't know.
To check to see if this is possibly your problem, take the shower head off of the flex hose, and inspect the head inlet for trash. There may be some in there. Also check the kitchen and bathroom screens on the faucets for trash.
The only good news I have for you is ours eventually seemed to clear itself and we got hot water back.
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