What can I do if I am pregnant and restricted to lift but work requires lifting stocks?

I been employed for eight months at my job. I recently found out I was pregnant. My doctor gave me an employer letter stating no heavy lifting or not to work over 40 hours nor to be around any toxic fumes. I was told by a manager that I was hired as a stocker and I knew of the job description when I signed the paperwork if I can't perform the work I can't work there. I was then told that I was not currently under any restrictions until I bring another doctors note specifically telling the number of pounds I can lift. He said he don't know what heavy lifting is. Ten pounds could be heavy for me. I was then asked if I need to go home and I replied why. He said that if you tell me you can't lift the freight you can go home. He stated that he has no positions that don't require lifting. After I bring in another doctor note, he takes it from there.