Some people, and some of them happen to be managers, are not friendly at all towards the idea of pregnant employees. I've been on a forum with other pregnant women during my pregnancy, and there have been some good stories in just our small group - the worst is an employer who cut a salary employee's paycheck by $450 because she had some doctors appointments, and tried to demote her to an hourly employee, and was making some hostile comments about the laws that required him to keep her job for her during maternity leave. She wasn't looking for much at all in the line of accomodations, but he thought it was all a bunch of BS that he couldn't just get rid of her, that his business had to deal with keeping her.
Fortunately, she knew enough about her rights not to just accept it... but it's a huge issue, and you are in a very insecure position when pregnant, so far as jobs are concerned. You need medical insurance - a birth can easily run 10 grand without complications - not to mention paying for doctors visits, lab tests, ultrasounds, etc. before the birth. So, you need to keep your job. A new job (even if you can get one, pushing a pregnant belly in front of you into the interview room) is not required to help you out the same way as a job you held before becoming pregnant, and the medical insurance may not cover the expenses.
It may have come down to - lift these boxes, and keep your job, or don't lift them, and become homeless without medical insurance, with a complicated pregnancy. Sure, you can go to a lawyer and sue... and while that is going on, you are homeless without medical insurance - not an acceptable solution. And lifting the boxes might not do anything bad - it's easy to do more than you are supposed to - I overdid it during my pregnancy - knew I was doing a bit much (moving day), but until my body protested after the fact, I didn't think I was doing anything that might be dangerous. So many rules to being pregnant that you can't follow all of them, so many that you can bend, so many pregnant women who break them and have no problems, and you think you can do it too.
And maybe she could have gotten enough assistance to keep everything afloat - but it doen't take much of a problem to eliminate all your savings and a bit more, and make you homeless.