I want to know what this could be on my left butt cheek? Is this cellulite or deflation?

I am only quessing since I have no history, but I have seen defects like this from blunt trauma or injections to the area leading to fat injury then fat necrosis and then the deformity. If the area is not badly scarred down, then fat transfer could possible help. See a plastic surgeon who does a lot of fat transfer work. Good luck.

Hello dear!
Thanks for the pictures and provided information as well.
Based on your photos, you are a good candidate for fat transfer to fix your problem, and I think you'll get very good results. 
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 :)

Based solely on the one view that I can see the issue in, it looks like more of a deflation issues. This might be able to be corrected with a BBL.