Seen online if got air in system it would cause this.
Heater blowing cold air! Changed termostates. No help on 2003 Nissan Altima
If there is air in the system the waterpump will not pump the coolant throughout. You may have air in the system and need to bleed it out, could be a bad temp./blend door actuator which moves a plastic flap inside HVAC box when you adjust the temp. at the controls, could possibly be a clogged heater core, or your new thermostate could be bad. Especially if it's aftermarket. I've seen a couple new aftermarket thermostats go bad in a week or so after installing them.