Red Marks From Sitting Strange Red/purple Marks On My Middle Back, Stretch Marks? Scars?

Strange red/purple marks on my middle back, stretch marks? Scars? - red marks from sitting

A few weeks ago I noticed some marks / red purple in the middle of my back and feel a little hollow. At first I thought I was sleeping on my back and I did leave a mark, or perhaps sitting in a chair, they are still there, but horizontally, and about 6 inches long, 1 / 4 inches wide. Are stretch marks? I am a man of 17 years, athletic and very slim, but muscular. I have no higher in recent months and have not lost or weight. I do not think I've recently such a cause of deep wounds and reduces red ... Suggestions?