I have lines in my back. It looks like scars but I never had any injuries before. It keeps on showing sometimes lessening?

Thinned dermis. I agree sounds like stretch marks which is thinning/tearing of the dermis usually from rapid expansion of the soft tissues such as pregnancy, weight gain, weight lifting, etc..
Don't understand... ...2d part. May be striae, or "stretch marks" which can have no known cause or be associated with heredity, obesity, hormonal influences, corticosteroid or other related drug use, Progesterone influence, etc.
LINES ON BACK. Cant tell without seeing these, they sound like stretch marks, but with no picture, can't really advise.