Horse with "hives"?

7-yo- Appaloosa mare. Everytime she eats she breaks out. At first had lesions with fluid, those are gone. Now has lesions of all sizes and shapes that go away as the day progresses, returns when she eats. Have tried different feed, everything. Has been to Las Vegas for 1 year and has had some minor problems like this since moving here .No problems in Phx. Any Suggestions? They don't seem to itch or bother her at all.