Hi S. -
Can you think back to when during the year this occurs?
It could be hayfever/seasonal allergies. This happens to me at times in late Spring. It depends on your allergies and what is in the air when.
For instance, a mold allergy might be in full force for someone now with all the rain we've had recently or back in April. August is typically ragweed season too.
Talk to your pediatrician. If it is allergies,(s)he may recommend OTC Claritin or Zyrtec, an Rx like Singulair or an Rx nasal spry like Nasonex - perhaps even a combination. Be careful if you try Zytrec - it worked real well for me, but I took at night as recommended yet still felt drowsy when I would wake up.
My allergy cocktail consists of an antihistamine (usually Allegra but I switch to Claritin and occassionally Zyrtec in the summer) and Singulair. I've also needed a boost with a nasal spray during my bad allergy seasons.
Don't freak about an ENT ... I've learned medicine is more art than science and most doctors would normally do trial & error with some of these medicines before putting you through the hassle and expense of a specialist if they don't feel it is really necessary. Be sure to let the ped know how you feel.