Your not supposed to bend over your supposed to like squat to pick things up.. its probably your baby saying your squishing him/her in there... my son does it all the time he doesn't do it when I squat...
You could be over working yourself, I'm 20 weeks and have been doing a lot of bending and it hurts really bad when I stand up straight. I just would suggest rest and tylenol to help with the back pain.
I'm going through the same thing I think mines is from having the epidurial with My other 2 kids