Is reactive hypoglycemia common in a young slim person? i keep geting symptoms of it but tested negative for diabetes

No: Hypoglycemia is opposit to diabetes that is hyperglycemia. If hypoglycemia happens frequently in a slim person, it usually indicates the person might not eat enough. Anorexia may be the reason if the slim person concerns her/his body shape. In rare case, insulinoma can cause frequent hypoglycemia too. Consult your doctor to check it up.

Thin pts report RH: There are many case reports & anecdotal evidence that reactive hypoglycemia (RH) can occur in someone who is slim (http://ehealthforum.com/health/skinny-people-with-reactive-hypoglycemia-t265917.html), but hard to say "common". It is usually diagnosed in teenagers or young adults, or after pts undergo bariatric surgery. It is a risk factor for diabetes but RH & diabetes rarely occur simultaneously