Unusual bruising on spine. Have literally done nothing to cause a bruise. It just started hurting and a bruise appeared!

Bruise. At your age it's still most likely that you bumped or struck the area and just didn't realize it, causing the bruise. Alternatively, it could be a bug bite or, even more rare, this could be a sign of an underlying bleeding disorder. For now i would give it time and conservative treatment, but if symptoms persist or worsen then see a physician.