I'm 32 years old female with 3 children. i had back pain for 6 months on middle left back. what it could be?

Back pain: Back pain is not that uncommon. Especially if you spend a lot of time carrying children. Take some Ibuprofen and a yoga class twice a week. At your age a good stretch can go a long way. If it does not resolve in 3-4 weeks, see your doctor.

Many things: Could be ligament weakness from many minor back incidents, or could be kidney problem . Need physician examination.