While it could well be dust mite allergy, despite the negative test for DH, it could well be a case of false negative reaction in people already on gluten free diet or selective IgA deficiency.
The photograph is consistent with DH.
This look slike it might be dust mite allergy. Try seeing an allergist and getting tested for dust mit allergy which might be from your bed and sheets. Make sure you wash all sheets in hottest setting to kill all dustmites; warm or cold water doesn't kill them. And it's their feces which causes the allergy so washing everything often is important. Also, covering the bed with an allergen cover is important to 'trap' the mite poop that's in your mattress and create a barrier between your clean sheets and mattress.
I also like to keep a sticky lint roller nearby so that I can run it over my pillow and on the area where I sleep every night in between washings. This should all help tremendously.
If it's not dust mite allergy, see an endocrinologist who can check for an autoimmune disease, but that is extremely rare.