I'm 5 10 and I weigh 180 my friends but mostly my grandma try to poke or jiggle or tickle my belly why is this?

It could be that they are just playing around, maybe your grandma considers it a loving gesture, perhaps some are thinking you need to lose some weight and they think they are giving you a hint without actually saying anything. Who knows... ask them! If you don't like it, tell them to please stop doing it.