Anybody here familiar with HR laws and pregnancy?

Lauren
Here's the low down: 
My coworker and I are pizza delivery drivers on a military base. Our job is fairly easy, but requires occasional lifting. The only duty we've told our employer we're unable to perform is moving the 2-liters (which are in cases of 35 lbs that we have to carry quite a distance and do like 10 cases at a time, then rotate them in the fridge). They've been telling us we're required to lift them as part of our job and that if we couldn't, we needed a doctors note. My coworker went and got a note from her doctor stating that she can't lift heavy objects, but when she brought it into work they told her that would be considered light duty, which is something they don't offer. So now they're taking her off the schedule and I'm left with a decision to make. Do I continue to lift the sodas and risk injuring myself and my baby, or do I get a doctors note and get taken off of the schedule? I'm a single mom of soon to be 2 and I fully support myself and my kids, this job is my only source of income and I can't afford to take any kind of pay cuts.