Hormones do play a big part in weight gain. With your period stopping completely, that is an indicator of a possible hormone fluctuation which could be due to several things. You are certain you could not be pregnant? Have you had sex at all around the time you stopped having your cycle?
You should probably go see your gynecologist. There are some health conditions which can cause the period to completely stop. Three months with no period is long enough to be concerning. You may need a hormone test or something of that nature.