work constantly call me in last minute

i've been working in retail for just over a month and my scheduled shifts are 12 hours a week mon, tues and wed - however since i started this hasn't been the case- which is completely fine i don't mind being given shifts WITH NOTICE! however recently for the past 2 weeks i've been getting at least 5 missed calls a week asking to come in and if i don't they go on the group chat and say 'certain people aren't pulling their weight' this week i'm working a total 63 HOURS?? yet they still have the cheek to ring me every morning asking to come in 2-3 hours earlier than scheduled with about 20 minutes to get there- i'm on absolutely awful pay and they don't seem to ask anyone else other than me to cover and i kinda just feel like they're mugging me off at this point lol. I've spoken to the manager and said i'm not happy with the short notice and he apologised yet it still happens- it's not like they are short staffed but nobody pulls their weight- i'm a sales assistant yet do the same jobs as the supervisors whilst they sit in the office doing nothing whilst i run the shop on my own... for £5.40 an hour. Is this fair? am i acting brattish? i'm only 18 and i'd like to think i can have a life outside of work but at this point i feel like i'm waking up, going to work, going to bed and repeat.

i'm also not contracted to be on call 24/7 and was never told this would happen so is it really common for it to happen constantly or are they messing me around?