Why am I overheating?

When I am driving usually kind of slow the temp gauge goes to extreme hot but when I stop and check under the hood the engine does not feel any hotter then normal. I replaced the thermostat but that did not stop it. I also replace the radiator cap and the hot water goes into the overflow bottle now but will not go back into the radiator as the engine cools, so I have to add about 2 cups of water per day.