Thanks for the good photo. This looks like some mild pigment change, probably due to friction of motion in the area. This may become more pronounced as she ages. This is not of any medical concern. Skin changes that you should be concerned about would include but not be limited to: redness, swelling, crusting or flaking, itchiness, increase in greasiness or discharge from the skin, or growths.
I hope this helps. Thanks for using Petco Pet Education Center, formerly Petcoach.