My Boyfriend Who Lives With Me Is Concerned About A

My boyfriend who lives with me is concerned about a bruise on his right buttock which on Monday was black and has a large lump which is numb and the bruise seems to be turning colors and spreading. He does pools for a living and spiders are around a lot. He did slip on the stairs but not that hard Monday night, the lump that's on the bruise is getting bigger along with the bruise, he thinks it might be a spider bite. There is no crater, I am familiar with brown recluse and hobo spiders because 2 people I know and have been to the doctors with I have been there through that... What could it be?