When you say morning, is this only after sun is up? Does the temperature normalize in the afternoon? What about in the winter, is it colder in that room?

I would tend to go with GreenvilleVT. I think you have a crimp in the ductwork so you are not getting all the cold air through the vent or, you have a leak in the ductwork. So, in the morning, when the air handling is running, take a digital thermometer and see what the air temperature is of each vent. And also, see if you can feel a difference in the amount of air. You could take a sheet of paper, put it near the vent to see how far the air blows the paper down. If it is significantly different, then you have a leak somewhere.