This is a common area for midge/gnat irritation and the damage may have been caused by your mare kicking herself to dislodge the midges rather than the midges themselves. Alternatively it could have been caused by a horse fly or similar.
Covering the vulnerable area with a belly wrap is the best thing you can do for her. I sometimes hold a cotton wool pad soaked in witch hazel on bites to cool and soothe followed by a dab of Sudocrem which has a mild anaesthetic effect as well as healing and antiseptic properties.
Looks like a localised allergic reaction to an insect bite (my old horse used to get them and it looks similar) If you are concerned, you could send the photo to your vet and ask what they think.