Today I made cupcakes using the same recipe that I always do :
240g Butter
240g Caster Sugar
4 Eggs
240g S.R. Flour
A splash or two of milk.
I got 21 cakes out of this mix.

I baked them at 170 for 25 minutes.
After I had left them to cool, I found that they had shrunk
They have never shrunk before.
What do you think happened ?
Also, they were slightly concave on the bottom.
I don't understand.