This is your body working to keep your core temperature stable.
If you’re in a hot environment, your body needs to adapt to the increased heat and protect your core temperature from rising too much. It needs energy to do this, so it will generate glucose from your stores.
Your glucose rises rapidly because of the sudden change in temperature, if you slowly increase the temperature of your shower the response won’t be so dramatic. You may also sweat more and become dehydrated, which can also raise blood glucose levels, so make sure you stay hydrated.