This is your body responding to the cold shock and working to keep your core temperature stable.
A sudden and extreme change in temperature prompts your brain to put your body into an ‘emergency warm up’ mode to ensure your core temperature doesn’t drop too low.
Shivering and goosebumps activate muscles to generate heat, while vasoconstriction (narrowing of blood vessels under the skin) directs blood away from the body’s surface into the core, to protect your core temperature.
If you have been reducing carbs, the spike you see in the app is your body releasing glucose from your energy stores in order to fuel all this activity and keep your body warm and working.