I would take a second test in a day or two to confirm your result and rule out chemical pregnancy. Do it first thing in the morning when you get up so your hormones are strongest and your urine is concentrated.
False positives are rare; most are isolated to defective test designs or to chemical pregnancy, which happens when an embryo fails to implant and is ejected from your body a few days later. Some women get spotting when the egg implants, but having this happen so close to your period is unusual -- your body would have just shed the lining that an egg would want to implant into. Do you know when you normally ovulate?