Why Is My Middle Back Spine Hurting?

Hi I am a 44 year old mom of seven youngest is a year in a half and for the past several weeks my middle back spine and to the right of my spine has been hurting especially when I lift the baby with right arm, lying on my right side or turn my body to left when I am sitting or standing. I would say I am at this time 20 pounds overweight and also still breastfeeding (HAVE BEEN TRYING TO WEAN)