Is it stomache cancer?

Ever since i was little i get stomache aches all the time. Nothing impartucular would cause them. Just random most of the time, or when im stressed or eat a LOT of junk. But alotta the time random. They tried giving me medicine that would help if i was Lactose Intalerant but it legit made me puke... In my family, my grandfather had cancer. Im very thin, and very tall. I had Pectus Excavatum & i have Marfan Syndrom. Im female, & 15 years old.