She kicked me in the "sac" or "sack"? Reference is to the testicles
Sack is short for nut sack or nutsack, meaning the scrotum.
(In case it needs spelling out, the allusion is to a bag containing nuts, i.e. testicles.)
A word spelled sac also exists, which Oxforddictionaries.com defines as
A cavity enclosed by a membrane within a living organism, containing air, liquid, or solid structures.
The latter spelling is mainly used in a life-science context.