Toddler only coughing at nap time??

[Wugz22]

This is driving me crazy. For the last 4 or 5 days, my toddler is waking up prematurely from his nap coughing. He wants to go back to sleep and tries while I pat his bottom, but the cough won't stop until he is upright. Then, he doesn't cough at all for the rest of the day! No other symptoms of illness, what is this?

 

[_Meep_]

If he's sleeping on his back, a secretion of some sort (mucous, saliva) dripping down the back of his throat would be my guess ...