Yes that is normal and what you did was a bit daft as you could have been badly hurt. Never remove the coolant cap if the engine is hot, or even warm. The water system is a pressure system. The warmer the water, the higher the pressure. Water normally boils at 100 degrees C, but if it is under pressure, the boiling point rises and can go up past 110 degrees C. I don't need to point out how badly scalded someone could get if they came into contact with such high water temps.
If you did suspect high pressure, then that could also mean low coolant level or an air lock in the system.