What's the cause of this flat spot on the inner lower part of my glutes? (Photo)

The flat area of the lower buttocks is a product of the pressure that is placed on the area by the ishial tuberosity when you sit down.  While the musculature in the buttocks is not affected much by prolonged sitting, the fatty tissue loses volume over time in response to the pressure.  Fat transfer to the buttocks can be helpful in improving this area.  Dr. B.

It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your buttock will determine, in many cases, the maximal volume per implant profile that you can accommodate. Liposuction of your lower back and fat transfer is also helpful as an adjunct to implants. Using “want to be” photos however are useful if simply provided to the surgeon.

The picture that you are presenting is common. Many women despite being in shape have these dimples. One of the most effective means of reducing and vanishing these is Cellfina. A new FDA approved non invasive no downtime device that effectively treats buttock cellulite and dimples. Our non invasive center, ENIC, can treat these very easily. The procedure is fairly non invasive with no downtime. 

Hello dear!
Thanks for the pictures and provided information as well.
Based on your photos, you are a good candidate for a small amount of liposuction and fat transfer, to fix your problem and improve your buttocks, and I think you'll get very good results.You can combine this with the other procedures, depending on your health and lab tests.
l recommend you to make an in-person consultation with a board certified plastic surgeon for a proper answer and to talk about your goals and anything you need to know.
Good luck :)