What can cause no period and weight gain

Right off the bat I’ve got a bunch of pregnancy symptoms. I have to pee a lot, I’m gaining weight, haven’t had a period in 50 days, I’m getting a lot of cramps. However I’ve been to the doctors twice and I’m definitely not pregnant. I am gaining a noticeable amount of weight and it makes me so sad because I spent an entire summer getting in shape. Does anyone know why this would happen???