Last night...
I finished the apron, patched the underarms on my Regency shift, moved the straps on my strapped petticoat, and took a pattern of my chemisette to make a new one.
I've worn the strapped petticoat many times, but somehow didn't notice that the straps went under my armpits. I guess that solves the mystery of what layer was cutting into my underarms on every dress. Oops :)
Now, to work some more on the chemisette...