Hi L.,
This could be just a growth spurt, but I noticed your other post as well. While neither of these issues sounds like a huge deal by themselves, it sounds like you feel as though something is going on with your son.
My daughters have both had strep throat at times that made them act "different" and concerned me, but had no "traditional" symptoms. Sinus infections can be that way as well. My point is, something like this may be going on, and he either can't voice the pain, or just isn't in any.
My standard advice is to go with your gut. Better safe than sorry, right? If this lasts through the weekend, you may want to make another appt. just to make sure all is well. Good luck!