I drank lots of milk as a kid without any problem. In recent years, it's a risky endeavor.
Does this mean I'm devolving??!
Or, you might have the gene(s) that turn off at adulthood.
Or, you might well have a totally different gene that's doing something else while it incidentally creates proteins that react against your regular lactose tolerance gene(s).
There are so many possibilities and so little time.
Best bet is to get aholt of some "lactase" pills, or drink milk without lactose.
Not all differences in what appear to be genetic processes are a evolutionary in nature ~ some of them aren't even changes in genes.
LOL!!