Hi. From the details given by you it appears that you have a good excercise habbit. These are the points I can make out from the history given by you:
Your back pain
is most likely related to your sitting for long hours or some probable injury
during excersise. It would be helpful to know if you have morning stiffness
in the back lasting for an hour or longer, which gets better after excersise or walking around; or if the back pain runs down the back of thigh or leg on either side. If the back pain is severe and recent onset, you need to stop the back extension excersises for a while and see the respone.
Maintain good posture while sitting at work, avoid bending forwards and lifting heavy weights till back pain settles down. If it persists, you need a formal clinical evaluation.
Shaking legs and arms - need more details to decipher the significance
Head ache and dizziness
if persistant need attention by formal clinical evaluation. But less likely to be related to the back pain.