Your thermostat is stuck closed.
Coolant flow is from the lower radiator hose to the water pump through the engine, out the thermostat and upper radiator hose to the radiator to be cooled and recirculated. The lower hose usually is reinforced so that it doesn't get sucked close under high demand and the restriction of the radiator. The upper on the other hand is not since it will always be under higher pressure. Because the thermostat is stuck shut, or is not opening properly, your pump is pulling all it can and it's flowing through the engine and the highest restriction and therefore low pressure area now becomes the outlet of the thermostat. And that is why it gets sucked closed.
Jeff
