1> If the temperature heats up beyond normal in the first few miles of driving; the head gasket(s) may be leaking. A radiator shop has the tester to check for a blown head gasket.
2> It also might be that you put pure antifreeze in the system; instead of mixing it with at least 30% distilled water. Pure antifreeze needs water mixed with it, in order to effectively transfer heat out of the engine.
3> If you didn't refill the cooling system at the RADIATOR FILLER NECK, or didn't recheck the coolant level in the radiator the day after working on it; there may have been air trapped in the system which worked itself out, and left the radiator very low on coolant. You can't trust the level in the overflow bottle, until you are certain that the radiator level has stopped dropping, and you fill the radiator to the very top.
4> The water pump may be defective; which would also have caused the heater to not work well.
5> The previous owner may have had cooling system problems; and may have put excessive amounts of sealer in the system; which could have clogged up the radiator and heater core.
6> The electric radiator fans may not be working; which is usually caused by a bad thermal fan switch. But sometimes the fan motor can be defective.