Its not the bedding, I had the same thing happen to me a few years ago when I had a couple of boas, I purchased the bedding from a pet store, not thinking about them having an ongoing mite problem, and sure enough when I put the new bedding in the enclosure the mites spread to everyone of my boas. Fortunately enough, I was able to cure the problem, get rid of the bedding, buy new bedding from another store and everything was fine. But the mites aren't in the bedding normally, it is coming from where you picked it up I am sure.