how do I address this without putting myself in it?

So I haven't told work I am pregnant for personal reasons. I've never had any time off since finding out and generally I haven't needed them to make any allowances for me. I plan to tell them soon to arrange my maternity leave (only need to give them about 2 months notice if I remember rightly). I am in the process of possibly changing location, so I don't want to get overlooked now I'm pregnant...so I'm waiting. Anyway my manager said she wants to get me to learn to work in the stockroom, I already know how it works - I just said I have a slight issue with it(i told her I'd explain tomorrow). I am a sales floor supervisor, they have 3 stock supervisors already and it isn't in my job description. I don't want to put myself in any danger being pregnant or have to lug around with heavy items when I'll be heavily pregnant (this is if I end up staying at this location). So what do I tell her instead?