Dark purple looking scars on the lower part of my back. Looks like someone beat me with a switch?!?

I havn't ever had any really bad trauma on my back, nor can I remember having any injuries, but in atleast the past four months I've had a large, long dark purple, scar looking mark on my back. It's horizontal and looks like someone beat me with a switch. Although it doesn't hurt, my back has felt pretty achy, sort of like it's about to snap under pressure. Anyway, recently I've noticed that there are more little marks appearing around the large horizontal one. They look very similar to the larger, original one but they're smaller in size and a little scattered, although all in the same general area. Anyway, this whole ordeal totally perplexes me because how could I have scar looking marks without ever experiencing any trauma or injury?! My only thought about what the cause could be is that there's some sort of internal trauma... What could this possibly be?! If this helps, I'm a 15 year old boy. Please help! Thanks!