I Started Wearing A Mouth Guard The End Of May.

I started wearing a mouth guard the end of May. About a week after wearing it every night, my bottom lip started to get red and swollen. I went on vacation and didn't wear it for a week. And it cleared up. Came home, started wearing it, and the whole process started again. Went to the GP and he put me on a weeks worth of prednisone and Claritin. Cleared up and was good for a couple weeks. The swelling came back with sensitivity on my top lip too. Went to another Doctor, who after talking to me said it had to be something I was coming into contact with. Decided it must be the mouth guard, so I quit wearing it. Everything was almost healed up, and I drank out a plastic cup with a washable hard plastic straw. All the swelling came back, and now it won't go away. Any ideas?