It's a dilemma Tabytha. It's probably going to be better for you to look for a different type of work - the type of ?lifting you are being asked to do in this job is obviously not agreeing with you, and it ain't gonna change. There is nothing to stop you looking for another job now - but you will need to handle this carefully and consider for example what kind of reference your employer would give, at the moment.
Ask at your doctor's and see if there is anyone who can help you talk this through with your employer and clear your intentions up.If you are signed off sick, within reason you can't be sacked, but that position won't last long, so you need to get a clear picture of how you can move on without losing entitlement to jobseekers allowance etc if you don't find work right away.