Saw a physio video that said flat mice force your wrist to twist in an unnatural position, which slowly wears down the tendons. Vertical ones keep your hand in that neutral handshake position. Makes sense the pain would drop off fast once your wrist isn't fighting the desk all day.
wait TWO DAYS?? that's insane. i had the exact same ache for months too and tried everything - ice packs, stretches, different hand positions. nothing helped until i got one of those weird looking vertical mice. i literally felt the difference in like 3 hours which is wild to think about. still can't believe offices are out here making people use those flat things that wreck your wrist.