Blood found in urine....What should I do?

Hi, I'm an 18 year old male living in London, UK. About 3 months ago I found a small amount of blood (brown at times but sometimes normal blood colour) in my urine. At first I wasn't too concerned and left it for a week or two but after seeing it a few more times I alerted my mum who brought me to the local district hospital where they did a urine test. The test was fine but they did find a trace of blood. 3 months on and there is still blood in the urine and this comes especially after ejaculation and I also have pain in my abdomen. I have had a blood test recently which came back as normal and I have also had a scan of the abdomen which also came back as fine.

It's got to the point where I'm no longer bothered about whether it's serious or not, I just want to find out what's wrong so I can start some sort of treatment.

What I basically want to know is what could be wrong with me and also what/where should I go next? e.g. should I be going to hospital asap.

Thanks.