I know, just... As near as I could tell, it almost seemed like it was buzzing on the next fret down, no matter how I tried to adjust my finger placement.

Except I had previously been able to do that note without buzz, so my current theory is that because my fingertips were really, really sore, the problem was perhaps because my fingers just weren't able to press right in the ways that weren't agonising, so my current approach is to take a day off from guitar practice to let my fingers heal a little.