Hi, Welcome to health care magic, Based on your query, my opinion is as follows regarding your painful indentation over your left buttock. It could be due to muscle atrophy or it could be muscle pain following indentation or depression in buttock muscle. Also,i will hope there is no ulcer but I will need to see that area to comment exactly what the skin is and how looks. But it has nothing to do with lupus disease, sjogrens sundrome, interstitial lung disease, ray aids syndrome and ulticurial vasculitis. And hence you should see a surgeon who will examine it and start treatment. Meanwhile,take pain killers if there is unbearable pain otherwise no need to worry. Hope it helps. Regards,