What causes pain in the abdomen and lower back?

Many. More info is needed . For females ovarian issues should be evaluated , pregnancy, pid, etc should be considered. Both men and women should have their kidneys evaluated, kidney stones, uti, etc.Men should have prostates checked. Lumbar disc disease etc can cause issues. Gi problems ibs/uc/colitis etc.See you dr for full full h&p and systematic work-up.