What do doctors check for when doing routine blood work?

My mom has had lower back pain (not bad) for about 2 months. She had an xray Monday and they called her Tuesday and told her they saw subtle abnormalities on the xray. They wanted her have blood work and urine test done to test for bone cancer (multiple myeloma). I guess my question is, is she had routine blood work 4 months ago and everything came out fine. She has never had any trouble with her bones and her back doesn't hurt everyday...just sometimes. If it was really cancer wouldn't it hurt all the time? And wouldn't her blood work have come back with some abnormalities? I'm just really confused right now! Please help!!