Milk

I buy Lausanne milk because it's a good product and the milk is always fresh. However since last year around November when I bought milk the sachets( I buy about 20 sachets and put it in my chest freezer) when I defrosted the milk it was sour. The other sachet when I defrosted it last just a day in my fridge. The expiring date when I bought it was like a week ahead. I have never had this problem and al of a sudden stated to get bad milk. Example I defrosted 2 sachets of milk yesterday the one was good I used it and I opened the other one last night used a bit and put it in my fridge which is on high and the milk looks like yogurt this morning. It's really a waste of money and of porridge or coffee when it's already poured in. Not impressed with the quality of the milk lately