Hi, and thanks for your medical question on eHealth forum!
Take a blood pregnancy test and if it is a three month pregnancy, then your gynecologist can also do an ultrasound abdominal way. Other than that hypothyroidism, PCOD and hormone imbalance can also cause pregnancy like symptoms along with weight gain.
Since a confirmed diagnosis cannot be given on net please discuss these possibilities with your gynecologist once and get yourself investigated accordingly. Hope this helps. Take care!