Have you had your thyroid tested? I had a friend in school who ate carefully, was very active, came from an average size family, and just couldn't get rid of a bunch of extra weight- if I or her sister had kept the same diet and exercise routine, we'd have been bone thin. We encouraged her to talk to a medical professional, because really, she was eating too few calories and doing to much exercise not to be losing it. She talked to a new doctor about it, and the new doctor got her thyroid tested. Turned out she had hypothyroidism.