Why is my Sun tea going sour in the refrigerator..

I just made a pitcher of sun tea yesterday and brought it in immediately put it in the refrigerator, yet today, the first glass I got of it, its sour. I use 3 black tea bags and 3 green tea bags and sweeten it with Splenda.

Any ideas why it sours?

you probably need to mix it before you drink it.. make sure you keep it covered next time.