Rancid tasting Fanta

I recently bought a 2ltr Fanta from Value Mart in Rooderpoort. Upon opening it, it seemed fine until I poured it in a cup and it was flat and had a rancid taste. I am now dreading opening the other cold drinks, as I have no idea how it will taste, and I was the first one to open it and break the seal. Bar code is ***.