The corner of my mouth hurts after eating a pineapple?

So a few days ago I ate a pineapple (and I ate the white middle parts too) and I guess I went a bit too far because my mouth was sore after that. But one corner healed with no problems a day later, the other one though - it still hurts 4 days later! It hurts to open my mouth all the way and slightly stings when touched. Why did it not heal? Is it some kind of infection? What should I do and how long will it take for it to go away? Thanks!