Car heats up after 30min long hwy drive, why?

Ok, so I have a car a 1999 Honda Civic DX Auto. It will run for days, weeks with no problem at all. Not a single overheat but when it's a cold morning. The gauge will not go anywhere near half way. It stays on C all through the cold day. Normal days its at half. After hitting the hwy at average 65mph for about 35min and letting off the gas. It will raise all the way to the H and start smoking. So i have to sit on the side for about 10min and put in more coolant. Its done that both times i did the long hwy drive.

Thermostat is faulty.

It's jammed 1/2 open. So the car is slow to warm up, but then it won't open all the way once the engine is under heavy load, and it starts to overheat.

If there are no leaks, you have a bad head gasket. There are only 2 places coolant can go, on the ground, or into the engine.

Id check the thermostst sounds sticky