That's not "tucked up".
Hard to judge from one photo so this is guesswork really but my spidey senses are saying it could be a sacroiliac / pelvic issue. The lack of topline muscle, her "jumper's bump", and the unevenness (ie. the fact that you have noticed this indentation only on one side) are all red flags.
Has she had her back checked recently or any kind of physio etc? Is she in work at the moment? Is she stiff or sore behind?
Did she lose muscle on the back end when she lost weight, or did she not have the muscle in the first place?
Would be interesting to see more pictures! Can you get a full-body side-on shot, and one from directly behind her when she's standing square?