Got $100 check in mail; said I "won" Yahoo contest; EPI Fulfillment - is it legit?

I received a check - no explanation for EPI Fulfillment - called bank who referred me to another # - that number referred me to a 3rd phone # - to see if check was legitimate. I was told that I had "won" first prize in Yahoo Shopping Spree - actually my grandson entered it - a "pop-up" that occurred when he was playing a game on Yahoo. My concern is that it's a scam to get people's bank account #s and signatures. Can you verify if this is truly legitimate? Thank you.