Having both back pain and stomach pain together, especially with fever, is something you really should not wait on. It could be a few different things, some minor and some that need treatment, so getting a proper check is the only way to know for sure. I would bring yourself to the polyclinic or a doctor as soon as you can, and if the fever is high or the pain is very bad, go to A&E instead of waiting. Postpartum issues can sometimes be easy to miss because we are so focused on the baby, but your health matters too, please take care of yourself.