Flat Lemonade

Well, long story short

- We bought a 2L bottle of Lemonade from Coles yesterday
- Put it in the fridge, like normal people do.
- My older sister drinks the lemonade like a b****h, then puts the bottle (half-full) in the fridge
- Now, I take the bottle out, drink it AND ITS FLAT!

And Im like, WHY WOULD IT BE FLAT AFTER A DAY FROM BUYING THE FREAKING BOTTLE?!
DID MY SISTER LEAVE OUT THE FREAKING BOTTLE FOR LIKE 5 HOURS OR SOMETHING?!