Halfway through my antibiotics for kidney infection and today my left flank has a dull ache. Could I be getting worse? Or is this something else?

Can't say. If you are not running a high temperature and the symptoms you had previously are better then it is probably a coincidence or something else but drink lots of water and take the meds and watch for any other symptoms. If gets worse then see a provider otherwise may watch for now. Water and the meds are crucial. Juices are good as the citrus acid helps to clean the urine. Hope this helps.