It's almost certainly your milk , try another.
Or gently pasteurize your milk first.
This is what's in your milk :
Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants and humans. A member of the genus Bacillus, B. subtilis is rod-shaped, and can form a tough, protective endospore, allowing it to tolerate extreme environmental conditions.
It is what reacts with lactose and creates butyric acid , I get it in my raw milk cheeses as well if aged too long , luckily I make mostly younger or mold ripened cheeses.
Your cheese may not be acidifying properly.