What Causes Severe Lower Back Pain After Taking Amoxicillin And Clavulanate?

Hello I have a quick question, I was prescribed with amoxicillin and clavulanate potassium tablets 875mg/ 125mg and told to take one by mouth twice a day with meal. Once I started taking them I've been getting really bad pain in my lower back, could it be a side effect or my kidneys or something? My dog bit me 1 day ago in my lip and caused a laceration and they had to put stitches on it. My last tetanus shot was on 1997 should I get one now? And also my lip is swollen could it be infection or because of the anesthesia they put on me to clean and put stitches on my wound?