This is pretty much me in a nutshell! I have always eaten carbs and gluten foods my whole life, I'm not 21 and in the past few months have noticed that n* feeling after eating pasta or toast or cereal. Yet these are the things I love! I too am underweight and have no desire to cut these foods out unless I know for sure I have an intolerance. I suggest seeing your doctor about it. I spoke to mine and she tested me for celiac disease, which I didn't have, and told me to write the specific foods I ate that made me n* or full/heavy right after I ate them so we can start eliminating what I potentially could be intolerant to. Maybe keep a diary and track what you eat and if it made you feel strange and bring it to your doctor. Good luck!