A "mojo bag" denotes one of the many colored bags filled with trinkets and knick-knacks that orginated as a good luck charm of sorts in the voo doo practice.
It gained its sexual overtones from 20's blues singers due to the fact that a mojo bag is supposed to be kept on your person where no one may touch it lest they drain its potency. Men customarily wore it under their shirt while women, on the other hand, strung it off their waist so that it dangled under their skirt. So, in effect, when you claimed you touched a girls mojo, in essence you were eluding to the fact that you had your hand up her skirt.