Can be. You should repeat the test when you are not having your period, to be sure that a lesion is not missed.
SEE YOUR MD ASAP. Blood in the urine with/without any pain, you need to see your doctor asap, you will need blood and urine test, a cat scan of your kidneys and consult with urologis if indicated, to find the cause and treat the possibilities are you may have a kidney ureter or bladder stone, kidney or bladder infection, glomerular disease like glomerulonephritis, chronic kidney disease and tumors including cancer.
Yes. All can be associated with lupus.
They found traces of blood in urine, but I was told not to worry by the nurse. What caused the blood?
Hematuria. Blood in the urine is never normal although microscopic blood in the urine is somewhat common. You should see a urologist and have a complete work up do hematuria.