Hi, My son, 20 yrs. ha been experiencing a burning and a lot of pain when urinating. It is only happening occasionally, and notices it when his urine is more darker than usual - but not consistently. He feels like he wants to keep urinating, but the tank is empty. He is not sexually active and is healthy in all aspects. He did have a grade 2 reflux from kidney to bladder when a baby. Any ideas? Thanks