Hello and welcome to HCM.
Thank you for your query.
If the UTI is this persistent, it could either mean you are resistant to most of the antibiotics prescribed, and a high grade antibiotic would be required to treat you of your condition; or it could mean there is a source/underlying condition that could be causing this.
should indeed have helped in finding the cause, a normal result and symptoms still persisting is surprising. An ultrasound
Also one of the reasons why burning sensation occurs along with a UTI is because of the loss of electrolytes. Thus requiring correction.
Please consult your doctor on doing a thorough blood work, and check all electrolyte levels (sodium, potassium, calcium, phosphorus, etc.)
I hope I have succeeded in providing the information you were looking for. Please feel free to write back to me for any further clarifications. I would gladly help you.