In the morning its mostly cold due to the fact that Anemia is usually one of the most common reasons for cold intolerance, especially if your extremities feel cold all the time. Its colder in the morning and since the pool was out at night it also is cold, but if you wait tell the afternoon it gets warmer so the pool won't be as cold as it was. In conclusion the water changes from the morning to the afternoon due to the fact that the sun is shining on it and making it hot.
I just fixed it up a bit credit to the first comment