What Does A Dark Mark On The Back Indicate?

my daughter has marks across her back that are darker at times. there are about 6 lines that go all the way across. it looks like she was sitting on an outdoor lounge chair, to describe the marks. they are thin red lines. the skin is a little raised where they are. she just told me they have been there about 2 months. no itching or pain goes with the marks. any idea what could cause it?