You have kind of an interesting problem, and ask some very good questions. Since there is no obvious answer from what you have described thus far, I would recommend that you make an appointment with an otolaryngologist
(ENT or Ears Nose Throat) physician to have them evaluate your tongue. If you don't actually have the bumps when you come in to see them, they may ask you to come back once they show up again. With that being said, to the best of my knowledge eating different foods while a intra-oral wound is healing should have no affect on developing a food allergy. If you do happen to have developed a food allergy, I would think that it actually doesn't have anything to do with the piercing. There are a number of different questions that would be helpful to ask which I am not able to in this forum however, such as how long the bumps stay for, exactly what they look like, whether they are able to be scraped off, and whether you have these lesions anywhere else in your mouth. In general, if something comes on quick, and also goes away rather quick, it is usually not something bad like a cancer. Sorry that I can't give you a better answer, but it is not really possible without being able to examine and question you more. Try to get in to see an ENT when the bumps are actually present if you can. Best of luck!