What Causes Slight Vaginal Bleeding?

im only 12 but i have breasts and pubic hair and for the past few days i have had a small amount of blood in my pants is this my period or is it a urine infection? i have had no pain in my stomach and thats why im unsure i have never had my period before and i would like medical advice on what to do if it is it thanks, a confused 12 year old x