Haematuria is blood in the urine. Infection is one of the most common causes of haematuria, and obviously would need treating if the test is positive. There is also a possibility of a low-lying placenta that is giving rise to this symptom. A mid-pregnancy scan (around 20 weeks), should have shown the position of your placenta.
If the symptom persists, or an infection is not found, you should ask to be referred to a consultant for further investigations. Your GP can arrange this for you, or you can phone the hospital yourself and ask to go in. An obstetrician would be able to check for other, much rarer, conditions that can cause haematuria.
I would like to reassure you, though, that the most likely cause of your urine test finding is an infection, which can easily be treated.