If you have an outdoor shower and you leave the hot and cold water on you could be getting a mixture of the two. I would check that first. Then check your hot water bypass valves and make sure they not set to bypass.
What type of water heater do you have? Suburban, Atwood, Girard, etc? Tank or tankless?
Why would the shower run out of hot water, then immediately checking the kitchen sink it's still scalding hot.
The kitchen sink may be closer to the water heater than the shower. The difference could possibly be the quantity of water in the pipes to the shower vs to the sink. You may also could have a crossover valve not closed all the way, compounding it somewhat.
See both these threads as I know you have a Suburban from another thread you have:
2011 Flagstaff 831 RLBSS
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