It could be almost anything. I am not a vet, and cannot give medical advice. Were this my horse, I would call the vet and see what s/he recommends. There are probably a multitude of possible causes, some of which would require immediate attention. It sounds as if you are providing temporary relief, but not something that can cure the problem. Antibiotics might be necessary, either to clear up or prevent infection.
It is never the horse's fault