Heavy lifting during pregnancy?

So I work in a restaurant kitchen where a lot of my work requires me to do heavy lifting. Like 30-50lbs boxes, heavy equipment, etc. My manager knows that I am expecting but so far i've been trying to tough the work out even though my lower back is in so much pain. What is the max you guys think I should be lifting, and when will I need to tell my manager that I need to be more careful about lifting things? Anyone else have a job where you are on your feet all day doing manual type labor?