I've been having some pelvic joint problems (started around 12 weeks) and when we went home on hollies i had to do some (brief) lifting of our bags which weighed 12kg and 15kg. The physio was HORRIFIED and told me that was much too heavy - but that's with an existing joint problem (caused by, as Ryn pointed out, the relaxin). However, I lugged around a much heavier suitcase (20+kg) on several occasions during my first PG with no ill effects whatsoever.

So I really think it's an individual thing and the moral of the story is listen to your own body and only lift what you feel is well within your capability, without pushing the limit.