Localized tissue reaction from an abscess can cause tension in the tissues, which can turn the lip inside out like that. It looks pretty serious. I also know that when a portion of the mouth is made to stay open like that, then additional problems develop due to drying of the mucous membranes lining the lips, gums, and mouth interior. This looks like a trip to the vet to me, before things get worse. There is a vet (MWheelock) on the forum, perhaps he will chime in, if he sees your thread, or perhaps you could send him a personal message?