Why do people say I'm pretty when I know I'm not?

Okay so like I was at my friends house and my friend's house with like 9 other friends and me and my friend Mandy were alone in the dining room and I think I was fidgeting with my fingers and then shes like''you're really pretty jenny''(and no shes not bi)and I'm like''please Mandy,your pretty'',and its wierd because I know I'm not pretty,but mandy never lies ,so what is it? and also at the same afternoon my friend christie is all like''you have such a nice body''but i dont and today my aunt carmen was talking to my grandma while I was washing my hands in the restroom and I overheard her saying''you know jenny is really pretty''and yeah shes family but still?are they crazy or what?