Q: Cooling system issue

I recently replaced a faulty radiator and blown head gasket in my vehicle after an overheat and some diagnosing of symptoms. The car ran fine after repairs, but I noticed that the Temp Guage continued to show that the engine was running cool. In addition, it was using more fuel than usual and not heating up the cabin as it used to. Checked the Thermostat and, sure enough, it was faulty and stuck in the open position. Replaced the thermostat and its housing unit, refilled the coolant and attempted to ensure that all the air was removed from the system. However, last night after running the engine for about an hour, I heard bubbling coming from underneath the hood and coolant was spraying from my reservoir. I carefully removed the cap to relieve the pressure and it continued to bubble inside. I also removed the radiator cap to relieve any pressure there and coolant just came spraying and "boiling" out. What could be the issue?
My car has an automatic transmission.