You probably just have air in the cooling system. Once the car cools down it may "burb" or it may not. This happens sometimes and usually a couple of start ups until the temperature begins to climb (allow to cool between trys) will remedy the problem. Another way to deal with it is to SLIGHTLY loosen the thermostat housing while filling the cooling system. When the coolant begins to drip from the housing snug up the fasteners. That should do it. Keep an eye on the temperature gauge as you obviously have been until everything checks out.

Randy