It could be several things. Here is a short list of what you can check easily: -your level of coolant is too low (check). If so, you could have a leak somewhere.
-your thermostat is stuck open (with engine cold, turn on your car and grab the upper hose that goes from the engine to the radiator; you should not feel any coolant going thru it until your car reaches normal operating temp; if you do, replace thermostat).
-your heater is clogged: check the inlet and outlet heater hoses with car turned on (at normal operating temp) and heat turned on. They should both be hot (the inlet being the hottest of them two).
If the outlet hose is not hot or just luke warm, your heater is clogged and you'll need to back flush your cooling system or replace the heater.
-your heater control valve is not functioning properly. Feel your inlet heater hose after the valve: *cold? If your valve is vacuum operated, check vacuum line. If it is electronic, check connection and voltage. If line or connection okay, replace valve.
*Hot? Valve is working fine.
Now the problem could also be one of the following; these are not easy to diagnose though, and a manual would help a lot at this point: -blend door actuator defective
-broken blend door
-weak water pump (is your car temp a little higher sometime than it used to be?)