Do you feel at all unwell? The first time I had a kidney infection I mistook it for a strained back for a few days (I didn't get cystitis first and had no other symptoms). However, I very quickly lost my appetite, got a high temperature and got very sick/dizzy etc., and ended up at the doctor (and only mentioned my back pain when he asked as I really thought I'd pulled a muscle!). The pain is different for everyone; my second kidney infection felt like stomach pain rather than back pain, so no two occurrences are necessarily the same. If you have any other symptoms at all or feel remotely under the weather then go to the doctor, but if you feel as right as rain then it's probably more likely to be muscular.