If a radiator gets slushy, it will do exactly what you describe. Coolant in the block will circulate OK Generally, but slush will block up in the heater core, and when the thermostat opens it pushes the slush to the other neck of the radiator and blocks flow. Then coolant in the head will boil...

When I was a kid I had a car that I dumped and flushed the cooling system and drained and filled with 50/50 mix. I didn't get in as much coolant as the thing was supposed to hold, and I later realized that a lot of plain water was left in coolant passages and I ended up with a weaker than intended mixture.

As long as nothing froze solid I'd bet you didn't do any permenant damage.

Also straight anti-freeze doesn't carry heat as well as 50/50. My dad had a truck that a shop overdid the mixture and it overheated. We were puzzled, but putting proper 50/50 mix cured the problem.

 

WOW, the guy has an overheating problem and everyone is tellin him he's got a cracekd block already. Take a breath people. Check the basics....
Does it have enough coolant in the radiator?? No--add more. As stated, if coolant is low, there will be no heat and no cooling capacity.

Have you tested the coolant to see if it's mixed for cold enough temps?? (if no, then you MIGHT have a problem with breakage)

Even though you replaced the thermostat recently, it could still be bad. I've installed new ones that were bad out of the box.

And YES, under certain conditions to much anti freeze could overheat an engine. Coolant alone does not have the ability to dissipate heat well enough under high heat or high engine load conditions.
Check the basics, diagnose the problem and go from there.

 

and---when it's really cold, 50/50 just ain't good enough. Up here we mix ours to 70/30. Not uncommon at all for us to see -48°C (-55°F) and antifreeze that only protects to -35 doesn't do us much good. But.....that's about the limit, 70/30.