It doesn't matter. Stop drinking. Sour taste is due to the fermentation of lactic acid bacteria in milk. If the appearance is the same as fresh milk and there is no separation of solute solution, it can be made into yogurt before drinking. The simplest way to make yogurt: find a clean and deep microwave bowl, scald it with boiling water, add milk, and then add a small bag of yogurt (the kind sold in supermarkets). Cover the lid, put it in a rice cooker with water, heat it to 40 degrees, and change it to heat preservation state. Yogurt will be made in about 6 hours, and it tastes better with sugar. If you like, you can add strawberries, diced pears, yellow peach pitaya, etc. to make a fruity yogurt.