My stepmother's knee has a bag of fluid.

She and my dad were in a very bad car accident on the 14th of last month. My dad ended up breaking his neck and three ribs, he was in the hospital up until the 2nd of July, and then sadly he passed away. My stepmom broke her arm and was hit and bruised all over the place but her knee has had this bag of fluid on it, I've never seen anything like it. The doctors only told her that it wasn't broken but it looks concerning. Any idea of what may be causing that? I've included a video and a picture of it moving around...

She and my dad were in a very bad car accident on the 14th of last month. My dad ended up breaking his neck and three ribs, he was in the hospital up until the 2nd of July, and then sadly he passed away. My stepmom broke her arm and was hit and bruised all over the place but her knee has had this bag of fluid on it, I've never seen anything like it. The doctors only told her that it wasn't broken but it looks concerning. Any idea of what may be causing that? I've included a video and a picture of it moving around...