max weight to carry?

Yesterday I took the garbage out and didn't feel like it was heavy...then started to have some cramps and back pain and got really scared! 
I have been doing grocery shopping and carrying most of the stuff and unfortunately my husband is working really busy to help me. I am not able to wait for him to come home and help me with this stuff. What should be the max weight that I should carry? I am 34 weeks ahead btw.