As stated above, before taking it anywhere to be checked, check the antifreeze level yourself.
I had an old Chevy snow plow truck that had a leaking radiator in it that I attribute to never having flushed the cooling system or changed the antifreeze in, that wouldn't put out sufficient heat when the radiator had lost enough coolant. Fill up the radiator, and it had great heat.
So check your coolant tank. It will have low and high marks on it. Even if it's not below the low or fill mark, fill it up to the full mark and see if it makes a difference.
Bill