Hi- I'm a junior doc so may be able to help in some way.
Usually when there is blood in the urine, there is a problem in the urethra (drains from bladder), bladder, ureters (from kidneys to bladder) and kidneys.
There are many many causes, including kidney and bladder stones, UTIs (which may or may not travel up to kidney like you mention), people who need lots of catheters in and out, inflammation of the kidney, foreign bodies, trauma and certain side effects of medication (assuming because you didnt mention you're not on them?), autoimmune diseases and other rare diseases of kidney, thin blood (coagulation problems) and of course, in the very rare instance: cases cancers of the kidney and bladder (cancers can be attributed to almost every symptom out there if you look at all of the causes of something). Also eating beetroot, weirdly.
You mentioned your kidney tests were OK, so GPs have to refer to urologists (they look at the structures further down than the kidneys) unless they are are suspecting diseases of kidneys first (then you would go to nephrologist). The GP is likely to have examined you to feel for any kidney masses which otherwise may have led him to refer to the kidney doctors. It may be that the urologists (well I don't know if the GP has. organised.) send you for an abdominal USS to look at your anatomy..eg, are there any stones there? You mentioned you get a slight twinge sometimes. Having said that, kidney and bladder stones may pass and may not then be detected on a scan! Or a urologist may decide to do a camera test, to look at your bladder, which is a cystoscopy. The GP has to go through the process of referring you to a urologist due to the government guidelines, as scarey as it may seem to be referred to a specialist.
I'm afraid I can't really comment on your individual case never having met you or examined you, but I hope some of this explains what it is happening. Do let us know what happens.