Need advice

Hi, so I'm 21 and 11 weeks preganant, I work as a checkout cashier in a diy store... But I'm finding it increasingly hard to do my job as its requires standing in one place for 7.5 hours a day, customers are stressful, and involves moving some heavy items across the conveyor belt. I spoke to management today and they said there's nothing they can do just try and stay calm... Any suggestions? Would doctors give a note so I can work a different department?