What could cause a crack in the drywall where the wall meets the ceiling?
You indicate that you have been in a drought since October.  Combine cooler weather, better insulated roof, and heat being on; the structural framing of the house may have shrunk.
I experience some "seasonal" cracking and shrinkage in my home.  See what it looks like in July/August, if the condition persists, consult a local experienced contractor.