Milk sour before expiry date

We bought milk on Monday or Tuesday night, a 3 litre, which is usually used up in our house within 2 days. Anyway, the expiry date was 30/8/2015, but it was sour by Wednesday afternoon. We hadn't even used a litre before it went sour. I usually always buy the local brands, and have never had this problem before. I took pictures of the milk as well, if need be.