The cause is much more likely changes in the microflora of your gut than your genes. (The human-lactase persistence allele is only one tiny part of the total picture.)
Lactose sometimes feeds bad microbes (because the microbes can use the di-saccharide bond for energy or because the the di-saccharide bond delays the absorption of the sugar, allowing it to feed bacteria residing in parts of the gut where starches and simple sugars are absent because they've already been absorbed upstream.)
In addition, microbes can produce toxins that interfere with the production or the effectiveness of the enzymes that cleave the di-saccharide bond.
The practical application of what I just said is that you might be able to regain your lactose tolerance by scrupulously avoiding lactose for a few months or a few years (i.e., long enough to starve out the offending microbial species). A good pro-biotic supplement might help.
Lactose sometimes feeds bad microbes (because the microbes can use the di-saccharide bond for energy or because the the di-saccharide bond delays the absorption of the sugar, allowing it to feed bacteria residing in parts of the gut where starches and simple sugars are absent because they've already been absorbed upstream.)
In addition, microbes can produce toxins that interfere with the production or the effectiveness of the enzymes that cleave the di-saccharide bond.
The practical application of what I just said is that you might be able to regain your lactose tolerance by scrupulously avoiding lactose for a few months or a few years (i.e., long enough to starve out the offending microbial species). A good pro-biotic supplement might help.