Thank you so much, ladies! I don't think it's a blocked duct, because I've had those before and the pain is more achy, whereas this is sort of stabbing. My nipples are completely normal-looking -- not extra shiny, pink/red, or white like descriptions I have found. And baby's mouth is normal; I've seen thrush in the mouth before, and everything in Robby's mouth is normal slobbery pinkness.

There's a suggestion on Kellymom about it possibly being nipple blanching. I'm going to pump from that side for today and if it's not improved, I'll call the doctor.
Thanks for your help!! Happy Friday!