Why are these guys doing this to me.. i didnt do anything to them?

ok well up at our naighborhood pool there are a group of my friends. some girls and mostly guys. well they always joke around with me like if i jump in the water "timber!" or "save the whale"! but im not fat becasuse i only waigh 113 and im 13. and my friends are fatter than i am. im not trying to be mean but im just telling the truth.... then they ignore me and try and piss me off.... but the worst part is that my girlfriends dont even help me out...... plz help me!