The most common cause of haematuria at your age is renal/ureteric stone. A small ureteric stone may be difficult to demonstrate on a USG scan. Usually this is associated with abdominal pain. Urinary tract infection is another common cause, but since you have already received 10 days of antibiotics it may have been dealt with. At your age a prostatomegaly is unlikely.
Next Steps
If this problem persists you can go ahead with a non contrast CT scan (if not done previously). Urinary alkalinizers may help occasionally. I would suggest you to visit a urosurgeon as sometimes a cystoscopy may be needed.