What Causes Pain Before And After Urination Along With Blood In Urine?

Hi! For the second time within two months, I am now in pain while, before and after urinating, and I have blood in my urine. My doctor tested a urine sample and did not find bacteria, and said to drink lots and keep warm. Last time this happened (also, without any signs of bacterial infection), my urologist did an ultrasound on my bladder and kidney and found no visible signs of obstructions, swellings or tumors, etc. Today I found visible blood and stringy, red bits in my urine (a little bit like one would expect from a period, except I can t have those anymore after having my uterus removed a few years back). Now I am concerned about what could be the issue, if apparently it is not an infection and it seems to be recurring and getting worse?