Ring and thread tale only working on pregnant woman's stomach?

Someone showed me that when you dangle a ring over your pregnant belly, it will predict your babys sex. she then told me that it wont move if you are not pregnant. it worked on everybody i know. But one day i was playing around and did it over my stomach, it showed i was having twins, a boy and a girl, I took 5 pregnancy tests and it says negative but still shows up on my stomach. i dont understand