What should I do?

I have a stressful and physical job. I am a manager and also every day requires alot of heavy lifting and moving stuff around. I am 22 weeks pregnant and it's becoming harder and harder on me. I have pulled a muscle last week and been to the hospital twice for bleeding after straining too hard at work. The store manager always says he will give me help which some days he does but most days I'm by myself and have too much to do. My dr doesn't seem concerned at all about this. I even tried to step down as manager and was told I cannot until I get someone trained but they won't let me have hours to train someone. Any advice from anyone? Am I just overreacting? Should I be able to still be doing all this work?