Lifting heavt object during early pregnancy??

I've recently started a new job and sometimes it requires me to help unpacking stock. A few times I've been asked to help move boxes and I obviously can't say no(because then everyone will know I'm pregnant) but I do try and be careful. But I can't help but worry and I always feel a slight strain on my tummy. I'm 5 weeks today. Isit a myth? Or should I really stay awa from heavy objects x