What can be the problem when my back is aching and I haven't lift anything heavy or on my menstrual cycle. I'm not over weight, I weight 117?

Idopathic back pain. In cases where there are no direct precipitants (eg. Trauma or excessive lifting), then the common causes are poor posture, especially if you are seated a lot of a chair/sofa without back support; or drive a lot without back support, or have a mattress that is too soft.