What Causes Stomach Pain And Blood In Urine Test?

i went to my gp this morning as i have had lower stomach and back pains for four days now i no its not a period as the pain is much sharper and worse me and my partner are trying for a baby but again if i was pregnant which tests show im not i would only be 3weeks anyway.... i did a urine sample and the nurse has said there is some blood in this (not that i have seen myself) why could this be?