Some UTIs take a few days to settle even on the right antibiotic, so this burning doesn't necessarily mean fosfomycin isn't working. New back pain alongside ongoing symptoms is worth flagging quickly, since it can occasionally mean the infection has moved higher up.
Next Steps
Go back to your prescribing doctor rather than deciding on another sachet yourself. She can confirm based on how you're responding.
Health Tips
Your report still shows pus cells and blood, so ask for a repeat culture rather than waiting it out.