My horses doesnt really like people getting into her mouth, or touching it. I never really thought it was from pain, and I still don't but yesterday I was working on breaking that habit. I held onto her upper lip like I was going to twitch her and I lifted it up and looked on the inside of the upper lip. There was a huge lump thing! It looks like a bubble, but it was hard and white compared to the rest of her lips. It was about as big as a marble. I don't have any pictures yet, but does anyone have any idea of what it might be?