By the exclamation mark, does this mean that "My foot!" is used as in English as an emphatic negative response?
Never heard about such an idiomatic phrase... no, I believe that someone has just hit his foot very hard and is in a lot of pain.
I think "My foot!" is something like "Akurat?", "Doprawdy?" or "Naprawdę?". When they say "My foot!" they're saying "As if! I don't believe you. Really?".
Of course, it's also something you'd say if you'd really hurt your foot.
It is true that English speakers use "my foot!" to mean "I don't believe you", although I have not heard it used in a long time. We might also say "you are pulling my leg".