Could it be that you stretched them more than usual while playing the Raindrop? You say you can stretch to the 9th, and that piece demands a lot from your pinkies.
But I get a real intense tension type pain in the back of my neck after I've played just one piece that requires a lot of concentration.
Yesterday I was practicing the next pieces:- Bach's 2 part Inventions (8, 7, 4, 1)- Chopin's Prelude op 28 no 15 (Raindrops)- Debussy's Clair de Lune (I am start learning it, I am only playing the first two pages)After the practice, my pinkies and my middle fingers started to pain.Today the pain is less, apart that all my fingers are not right like the rain (They are curve).Why could this happen?Note: my hands are not very big. The distance I can reach is at least to the ninth key.