Ok I recently replaced my radiator

Ok I recently replaced my radiator. My heat worked for about a month, now I have freezing cold air coming out it worked for about 2 days at high speeds but now at high speeds it still comes out cold.

I found a small leak on the center freeze plugg but nothing to crazy. Maybe a tablespoon work. I keep checking my coolant in my radiator(i check it while the car is running) and top off when needed.

my best guesses are: thermostat or my water pump.
I was thinking my head gaskets considering im losing coolant and not find any major leaks but I dont have white smoke from my exahaust and I dont have any extra fluid when I check the dipstick.

I do most of the work myself but when it gets really complicated I asks my boyfriends friend for help, These are the only 2 things I could really think of off the top on my head. Any help would be greatly appreciated!