Why do my sisters always shout at me?

ok, im 9 years old. at around the beginning of evry month my sisters are always swering and shouting at me. this happens for about two days every month. i do worse things somtimes but they dont do anything. but at round evry beginning of month even if i do a small thing they scream at me. please tell me why it is like this and they are 14 and 16 and i dont like it.