My Daughter Is 10 She Has The Whelps That Come

my daughter is 10 she has the whelps that come up on her body and they look horrible and they will totally disappear with in an hour with out taking any medication. They are white long whelps and can cover a large area and then go away as quick as it comes. She is now having joint pain and some swelling. knees hips wrists cant make a fist swollen thumbs, shoulders. Been in the hospital for three nights. Ran a bunch of tests and everything has come back negative. Any thoughts?