This can be due to muscular weakness of the back muscles which generally occurs because of long sitting hours to take care of the baby or /prolonged posture assumption (aide lying on one side) which leads to overuse of one side of muscles along with lack of use of the other side.Its very common after pregnancy and delivery, generally a great change of life style occurs in carrying a baby or caring a baby.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy +IFT +Heating therapy +some strengthening as well as stretching exercises will be helpful for sure.
Health Tips
At home.. Rest +hot water fomentation +pain killer gel +no long sitting /standing /travelling /walking +frequent change of posture. Avoid carrying the baby in your arms while standing .