Suspicion confirmed.

I forgot to give the imp his dose of Zyrtec this morning.  He was a complete and total butt-head all morning, then he had a dose of Benedryl (which calmed him down, for the most part), which wore off about 4:00 (and was re-applied at about 7:00).

I'm pretty sure he has pollen allergies of some type.  The pixie has been showing signs of such, too--not cranky and mean, but whiny and prone to tears at the least little thing.

I won't be forgetting the imp's Zyrtec in the morning. It's too bad the pixie is still too little for it.