Obviously

You place the necklace around your collar and a magical glow emits from the gems. You suddenly black out.

Coming to, seconds later, you know you're in a bad place. You're laying in a bale of hay in an alley a few blocks away from your cart. It's dark now and you hear people drinking and singing. You sit up and feel much lighter than usual. Strands of brown hair fall in your eyes and down to your shoulders. Your skin is much lighter now as well, and a quick look shows you're not in your clothes. You dart over to a window nearby and scream in terror as looking back at you is the girl you'd gotten the necklace from.

The necklace! It was gone. You somehow swapped bodies with this girl! It came to mind that yes, you were very pretty, but still, this was bad. You were lost in the city, nothing to your name but a blade and some rags for clothes, and you had no known home. You dip into a dark house. The dwellers you assume out for drinks. Inside you strip and inspect your body. You're very slim. Not over 100lbs. You're short, too no taller than 5'. Your breasts barely an A-cup but with very pert little nipples. You run a tiny hand over your sex and feel virgin maidenhood.