Hi welcome to Healthcaremagic
Since the Pain in the groin and back caused after the placement of mirena
, you can remove the Mirena IUCD and opt barrier method of contraception(condoms)... If the pain still persists then investigations like USG scanning is needed to find the cause for the pain eg. kidney
stone may also cause pain.. since you had already treated for the RTI ,do not take further anti biotics, remove the Mirena and observe for pain..