I got in a fight and now i really hurt?

im 15 years old, 5"10 feet tall, and 150 pounds. he was really fat, probably 5 and a half feet tall, and 250 pounds. we never have got along but today he jumped right in my face calling me names and telling my he was going to put me in the hospital so i told him to back off, then he pushed me to the ground and started laughing so i got up and called him a stupid fat *** and then he swung at me so i ducked and uppercut him then hooked him and he almost fell over but out of nowhere he punched me and i fell over again. i got up and grabbed him but he grabbed my wrist and started swinging me back and forth then he pull me at him and tried to do a bear hug but i kneed him in the nuts and then pushed him against a tree and gave him about body blows but he pushed me back and uppercut me to the ground and i was blacking out then he started stomping on my stomach until someone pulled him away. i tried to get up but my stomach hurt so badly i could barely move, so i laid there for about a half hour. now it still hurts and i dont know if i should go to the docter. what should i do?