Different bodies react differently to backbends. What does it mean when the body seems to be screaming, "STOP!" First, the body may be alerting you to an underlying problem, and in that case a health care practitioner should be visited for assessment. Alternately, the body may simply be experiencing the protestations of tight muscles. Some bodies naturally will backbend easier than others, but bodies of all levels of flexibility whose muscles are prepared can reap the rewards of backbending.
To work towards backbends begin by first sticking to a consistent practice of standing poses, twists, inversions, and forward bends. Twists and forward bends work to create space in the spine, and the standing poses and inversions produce a back that can stretch more effectively and maintain space. Try also to practice a modified Ustrasana (Camel Pose) by bringing the hands to blocks set behind the back, which reduces the likelihood of compression in the lumbar and sacrum. Yoga teaches students to be patient with their bodies, and though practice will probably not make perfect, it can lead to a happy, healthy back.