I am trying to figure out why my left butt cheek has been bruised for about 5 days with no falls or injuries?

A bruise (ecchymosis) can spontaneously occur on any part of the extremities or trunk without injury. If the size has not changed over 5 days, no tenderness on palpation and the color is now turning into yellowish green then you have nothing to worry. However, if the bruise progressively increases the size & the outer margin is still purplish red then you may see a doctor for a proper assessment.