Who do you call to determine why one room in the house gets really hot while the rest remain cool?

An HVAC tech could come out and check your ductwork. However if the room gets hotter only when you do not run your AC it is probably an insulation issue in that room. It is common for additions to have trouble maintaining the same temps as the rest of the house if this is an addition. Either way a good hvac tech could figure that out too.