Suggest Treatment For Missed Period And Weight Gain

Hi! I am 40 years old, am not pregnant, and have not had my period in about three months. I am healthy and of average weight, but have gained some 5 kilos in the last six months. I have not exercised as I should and have done regularly in the past, and life has been farily stressful due to a close friend's illness and death last Sept. My doctor is not too worried at the moment. Some additional info: I take 50mg of Ritaline for ADHD. Thanks for any ideas!