Usually people do feel unwell in the initial few days after quitting/cutting down a substance, due to withdrawal effects. That is, the body which has long been used to function in the presence of a substance, finds it difficult to carry on in the absence of the substance and needs time to adjust. However, your low back ache appears to me to be unlikely to be related to your cutting down smoking
. I would suggest that you consult a doctor and get yourself examined.