Feet swollen, gained 12 lbs. MAJOR HEALTH ISSUE PLEASE HELP?

Ok, so i went to hawaii last week and the trip was ffine my feet were fine everything was fine. I came back on the plane ride home and my feet swelled up big time, i got home after hours and hours my feet were still swollen, im 16 years old and im somewhat active. i noticed when i stepped on the scale i gained 12 lbs in one week, im really scared and i have no idea whats going on, can someone give me suggestions on what to do? my foot swelling wont go down, i put it in cold water and elevated it. this has never happend to me before