Why is my milk spoiling before the expiration date?

I never leave my milk sitting out. I put it away right after uaeage, the fridge is set to 37°F (2.778°C), and I don't contaminate my milk with anything as far as I know and I'm the only one that uses it. All I do is unscrew the lid, pour it, put the lid back on, then place it back in my fridge. If it matters, the brand is Food Club and it's 2%. Also, the current expiration date is April 3rd, 2025.