The spine is a complex arrangement of bones separated by discs which act like shock absorbers. Most back pain is caused by injury to these discs or the ligaments that hold the bones together.
By avoiding further injury, improving your posture and by taking pain-killing tablets e.g. Nurofen, most backache will resolve in a few days. If it does not, or if your back pain is also accompanied by pain down your legs or arms you should let the doctor check you over.