What Causes Joint Pain?

I'm having really bad joint pain, feels almost like it could be even in my bones of my hands, anemia and low magnesium. The magnesium has been low for the past two months. I just started feeling really achy and having joint pain in the past five - seven days. Saw my dr. yesterday and he thinks it's muscular. I'm worried he is overlooking something more serious based on how I'm feeling.