flushing a heater core

After finding ice in the radiator, I took the hoses off, and brought the radiator inside my house to thaw out. ( I wish I had thought of this sooner, as I do not have a garage.) I flushed out the block and water pump and flushed the radiator out in my bath tub. I also stopped up the bottom hose opening and filled it with water to check for leaks. It didn't leak.

I removed the thermostat and boiled it. It barely opened around 230 degrees, and I found plenty of stop leak inside of it. So I got a new thermostat to replace it and plan on re installing the radiator and hoses tomarrow.

Why would it start spewing steam out of the overflow after I removed the heater core? ( It didn't do this before) Also, should a new thermostat and new antifreeze solve the problem?

As to the ice in the radiator, there was no ice in the block, and I drained the coolant out of the block just to make sure I didn't have an expensive disaster.

Thanks