My cat has had this red spot on her eye....?

it's been about 3+ years idk exactly...it started out as just a little speck on her right eyeball and over the past couple years it's gotten steadily bigger. at this point it's all along the side of her eye and it's more of a long line along the side as opposed to the spot it was before. it looks like just red coloration idk exactly what it is. she doesn't seem to be affected by it or anything but it kind of worries me because it does keep getting bigger and my parents don't think it's serious but i don't know for sure. does anyone know what this might be? should i be worried or anything? if it matters she's about 8-9 years old but a very healthy, young-hearted cat.