I'll go for the air pocket. If it was OK just before the coolant change and not afterward, then it has to be something simple.
1. Check your radiator cap is sealing properly.
2. Burp it by turning the heater on full bore, take it for a short spin. Should notice the water level in the o'flow tank drops. If that doesn't work, jack it up (make reservoir side the highest) and run it with the cap off.
A good tip for next time is to jam an asprin (not soluble) in the thermostat to hold it open. When you start up, the thermo is open till the aspro dissolves and this will help expel any air in the system.