She is right, your bp could have had eggs, or you could have picked them up at a show, etc.
Do you buy your supplies from petland? Mites can be on anything you get from the pet store. It's not uncommon for pet stores to be infested with mites considering where they get their stock. I always advise people to put their bedding and whatever else they buy from the pet store for their bps in the freezer overnight to kill any mites/eggs that could be on the items.