if it looks like a real ring, and doesn't have some design on it like a bird or a heart, maybe its a purity ring. I don't know much about them or if you're supposed to wear them on your right hand... hmm.
but when I wear rings I wear them on my right ring finger. they feel odd on my left hand
It means she has a ring on her right hand. But seriously, it doesn't mean anything. The left hand is where your marriage ring goes, however, in today's time its pretty common for girls to wear regular rings on that hand too.
People sometimes put Promise rings there; Europeans often wear their wedding rings on their right hand; or she simply could like it there... it is her ring-finger afterall, and less suggestive than being on the left.