lump on back of knee

before DD went to bed tonight she said the back of her knee hurt. i looked and saw nothing, and soon after she fell asleep. Well 3 hrs later (about 20min ago) she woke up semi crying/whining saying the back of her knee hurt she kept saying oww.
So i got her to lay on her tummy, i could see there was a lump. So i asked her to stand and she said it hurt to stand up straight.... I could tell it was a large lump slightly red and it appears there is a small puncture in the skin in the middle.

Im thinking possible spider bite? idk.... are spider bites dangerous? (besides black widows i know)

could be a bug bite or staph infection. can you post a pic?