What Does Red Vertical Lines On The Stomach Indicate?

Hello, I have 3 about 4 cm long red vertical lines on my stomach right above the navel. They are not like real lines, more like markings in a shape of a line. I am a girl and 14 years old. They do not spread or anything and i have had them for a while now, though they are not so bright anymore. I just had my graduation exams. Maybe they are just stretches or something from stress?