Maybe this is an overly obvious question: Have you had the vet check her out? That swelling is in a lymph area, and since you suggest it’s frequent, probably deserves a vet’s opinion. Also, have you tried any anti-fungal etc products? The Eqyss line works really well on one of my guys who’s prone to allergies and fungus (we live in the deep south, the skin crud capital of the world). Usually contact dermatitis can be stopped if you remove the allergen. Since you know one of her triggers is the Back on Track, stop using it — perhaps switch to something that doesn’t cause swelling. Keeping the skin dry and clean is also important. Thorough grooming is usually far better for the skin and coat than too-frequent bathing or hosing. When you do bathe / hose, makes sure she’s dried quickly — not standing in the summer heat with wet skin.
Good luck and good riding.