Dear patient Back pain if mild to moderate severity is common during pregnancy due to increased abdominal size. Since you are 6 months during this time Ther is rapid increase in weight of baby which increase size of uterus and hence abdomen and puts strain on spine. There is nothing to worry about it as it subsides on its own within few months. Take rest for few days and do back excersises. Modify your posture while standing and walking so as to put less stress on tour spine. If it s severe and affects daily activities you can take tab paracetamol 500 mg thrice a day for 3 days for pain relief. Also add calcium supplements to your diet that will increase bone strength.all the best