The bone on my left arm is painful, i have had no injuries and dont think its broken. what else could be the cause?

The initial concern is very vague - so difficult to answer without a face to face visit with a doctor to examine the area. The pain can be anything from benign over-use injury to much more concerning causes (which includes infections, cancers, auto-immune disorders, etc). Any persistent pain should be evaluated by your doctor.

There are many potential causes for bone pain especially in long bones like the arms & legs. Instead of guessing a proper Medical Evaluation including appropriate imaging may be necessary to determine the cause of pain and how to resolve it.