I'm not very physically active. I have a desk job and then in the evening, I'm too tired to do anything. Why is my back sore?
To know what's causing your back pain, you would need to see your doctor, however, lower back pain is not just caused by lifting objects or moving the wrong way. This type of pain can also happen to people who sit for long periods, especially if their sitting arrangement isn't well suited for them.
Make sure that your work station is comfortable and healthy for you. Use a chair that is low enough that your feet can sit flat on the floor. If this isn't possible, use a foot stool for the same effect. Make sure that your chair has a curve in the lower part to support your back, the lumbar part. If your chair isn't equipped with the lumbar support, you can buy a cushion that does that for you, or you can take a rolled towel and place it in the curve of your back. Try not to cross your legs when you are sitting. Change positions often, try to get up at least once an hour to walk around.
If you use a computer, be sure that your monitor is at a comfortable viewing level so you aren't looking up or down, keeping your shoulders relaxed.
Francisco Kovacs MD, PhD, et al. A Comparison of Two short Education programs for Improving Low Back Pain-Related Disability in the Elderly. A Clustered Randomized Controlled Trial. In Spine. May 2007. Vol. 32. No. 10. Pp. 1053-1059.