Sounds daft but the inside of my bottom lips really stings whenever I eat or drink - anything fizzy is really painful especially. Even brushing my teeth - if the toothpaste touches the inside of my lip it stings like mad. There is absolutely nothing there to see but if I rub it with my finger it also hurts. Has been like it for a couple of weeks - any ideas??
If your mouth generally feels a bit odd, it could be thrush (candida), I've just got rid of it. Have you been on antibiotics lately? Inside of lip will be red, as will tip of tongue - pickled onion can send you up the wall! If it is, you need a mouth rinse from the doc's (but live yoghurt helps).