Blood in urine can be serious, so you do need to go back to your doctor and talk to them, perhaps do another test. It is pretty common for something like a kidney stone to cause blood in urine. These can get extremely serious may even be the cause of an UTI, although sometimes they can pass without notice nor problem. I'm not saying this is what caused the blood in the urine, just using it as an example because kidney stones are pretty common. As it is, do your best to not be afraid, and make sure you keep up with your doctor.
It could still be a UTI even if you haven't had any pain, you dont always experience pain with those. It could also be a more serious problem like a problem with your IUD, you should talk to which ever doctor called you.
Hematuria (or blood in the urine) should be investigated, as someone has already commented. The most common causes are urinary tract infection and kidney stones. It is possible to have either one without symptoms. My guess, though, is that you don't have a urinary tract infection, because your doctor would have seen evidence of that on the urinarlysis as well (white blood cells, leukocyte esterase, nitrates, etc) and probably commented on it. You could still have a urine culture, though, just to make sure. Kidney stones cause pain when they try to come out of the kidney and into the collecting system and move towards the bladder. When they're not doing that, they can be asymptomatic and cause microscopic (or unseen) hematuria. You can have KUB (abdominal xray) to look for radio-opaque stones or sometimes a CT scan to look for non-radio-opaque stones. Another common cause of hematuria in women is actually blood coming from the uterus, such as menstrual bleeding. Sometimes, it helps to give a urine sample while having a tampon in place, to get a sample that is purely from the bladder.
Worst case scenario, and least common, is a bladder or kidney mass, which can cause hematuria. This is worked up with cystoscopy and CT scan. If all of the other causes are excluded, this can be pursued, but I certainly wouldn't start there, especially in a young person.