the other posters have given you good advice. Jade refereed to a rodent ulcer this is very likely what it could be although they usually appear on the lip itself. my Sami had a similar bump that appeared on his eyebrow and it was finally dx'd as Eosinphlic Granuloma complex where rodent ulcer is also in the same family. his was traced to food allergies..namely chicken especially in the dry foods...it could also be a grain allergy also mostly associated with dry food diets, have you tried feeding your kitty canned foods? I also recommend you try grain free canned foods, its hard to get them to like these its like feeding broccoli when we've been eating candy....urrr
I will attach a photo of the sore my Sami had and I will also attach some reading and photos of granuloma complex and rodent ulcer and see if you can relate..please post a reply
http://img.photobucket.com/albums/v610/jlcutypie/Mobile%20Uploads/image_zps12e183a7.jpg
and this is my Sami when he had the sore(all better now)
http://www.medhelp.org/user_photos/show/274134?personal_page_id=473886