Logically I know you're right. I'm being loaded with extra shifts and I'm physically in pain right now, I can't refuse as we are understaffed.
Manager rang earlier and told me I'm doing all day tomorrow.
Oh that's not good. If you're in pain because of work you have every right to mention it to your manager, perhaps you could have a quick word with them to see if there is anything that could be done to help. Is being understaffed temporary? (i.e. is your work looking to hire someone?) If so, you could focus on the fact it will ease up soon and just try to get through the next few weeks or however long it may be.