I dont think youre being silly at all. They shouldnt make you lift these big boxes even when not pregnant. The boxes are also obviously not particularly easy to carry, so thats another problem, as anyone could hurt their back or something trying to lift them.... At the very least they should let you have smaller smaller boxes or one of those 3 wheel trolley type things to move the boxes with.
Ryn, your workplaces policy about lifting the boxes is stupid! I hope that you office manager takes this seriously and works out an alternative. Surely someone can help you out and move your boxes for you? I'm sure it doesnt take too long..

Also, I dont think you are worrying about nothing. I mean, I did some archiving at work yesterday and filled 3 archive boxes full of paper, then decided they were too heavy for me to move, so I ended up pushing it with my feet because I was scared to lift it! I think they weigh about 12kg when full, which isnt that heavy, but I dont want to strain myself, especially now, so I can also understand your concern with lifting stuff.