Chances are, one of your first passes in the neck area is going ATG. It is not uncommon to have more than one growth pattern in the neck area. You could have three different growth patterns, and a "swirl", on your neck. That's one scenario.
Some of the problems could have been caused because the Merkur blades were not right for you, and they were causing the irritation. Since you recently switched to Derby Extra's, and they are typically a more efficient blade for a lot of us, keep in mind that any change in our equipment will also necessitate a change in our technique.
I would bet that your problems will go away within a few days, as you adjust to the change. Patience is a virtue!
Merkur/Mergress, Derby Blades, Penworks/ST-24, E J/Super Badger, Mama Bear Soaps