My 16 month old has decided to quit naps out of the blue! HELP!

He was at home with my mom yesterday and she couldn't get him to sleep AT ALL. Made two attempts. Morning nap - nope! Afternoon nap - nope! She said he may have slept for 30 minutes (the entire day, eek!).
Today he is home with DH and the same thing is happening. Refusing to nap - screaming, crying, big, real tears!

What is the deal? We have given tylenol in case of teething pain (but that hasn't been an issue at all in the past and he has about 6 teeth so far).

Any ideas? No other big changes in his life right now and nighttime sleep is still good/normal.
ETA: His health is also good, he's not sick and just had a great check up at the doctor. He's in a great mood all the time, except when you try to make him nap. He is visibly tired though after being awake all day!