Anemia / iron deficiency

There are also other symptoms that occur with iron deficiency. Low back pain is one of the most common early signs. Iron is a catalyst to vitamin C in the formation of the connective tissue, collagen, that gives our tissues strength. The lumbar spine takes more mechanical stress than any other part of the body because so it is one of the first tissues to complain. 

http://www.olaloa.com/iron-deficiency-more-than-tired-blood.html

Also it says do not take iron supplements unless a doc says to because too much iron is really bad for the body and it's easy to get too much from a pill.