I am sorry to hear that you are drowsy all the time, even when you sleep for 8-10 hours, and that you are worried as to what might be going on. You mention that you have gained some weight recently which makes me think that there may be some obstructive sleep apnea
present, but ultimately I ma going to recommend that you make an appointment to be seen and evaluated by an Ear Nose Throat physician to get evaluated. they will be able to take a thorough history, look through your past medical history, and examine you. this will give you a much more accurate differential diagnosis, then my initial thoughts after what little information I have been given. Nonetheless I am happy to give you my thoughts as to what might be going on. Obstructive sleep apnea is a disorder caused by obstruction of the upper airway during sleep when muscle tone is diminished that causes decreased airflow, and ultimately decreased oxygen saturation in the blood. The brain doesn't like this, so you wake up, regain muscle tone and breathe fine until you fall asleep again. This becomes a bad cycle leading to non-restful sleep. the way to tell if you have obstructive sleep apnea is to get a polysomnogram (or PSG) which an ENT will be able to order for you if needed. Best of luck.