There are three possible case for milk carton become bloated.
Milk packages are made of paper materials, and in some cases, the fibers of the paper tend to soften and stretch over time during storage and transportation, and the bottom tends to swell.
As long as the package is not leaking, and is not bloating at the top, the milk may be consumed.
You may have purchased some milk and taken it home unrefrigerated, during which time the air in the package has warmed and expanded, pushing against the package and causing it to bloat.
If the package returns to normal when chilled in the refrigerator, the milk may be consumed.
However, if the milk is kept unrefrigerated for a long time, microorganisms in the milk will multiply, causing a risk of spoilage. If the milk has been kept unrefrigerated for a long time, please do not consume.
If the milk has been mishandled, for example by storing at incorrect temperatures, microorganisms in the milk can multiply, producing gases which cause the top of the package to bloat firmly.
In most cases, this is accompanied by changes in flavor and/or texture. Please do not consume.
