Morel-Lavallee Lesion.

Sheer injury, classically over the greater trochanter, causes development of a hemolymphatic collection at the fascial layer. Often misinterpreted as a “hematoma” these actually can be delayed in development and the longer they go untreated (compression and/or drainage), the more likely they will need surgical intervention.

Low-intermediate density oval shaped fluid collection that typically touches the fascial layer over a bony protuberance where there is a history of trauma. May take weeks to months to develop

reference: https://radiopaedia.org/articles/morel-lavallee-lesion-1