Abandon Kitten sick? or Not?

Okay this Kitten was abandon he/she is at least 5 months old and my mom boyfriend grandma found this kitten wondering around it was really skinny and scared Shes eating & drinking water.But if you touch her back she hisses I think its cause her bones are really weak he/she scratched me it didn't really hurt cause shes really weak we gave her some milk to. We are gonna take her/him to the vet soon were just trying to get some weight on her.We didn't want to give him/her to the pound to just suffer their so we are gonna keap her/him.we cant tell if its a boy or girl cause if you try to pick it up it sorta hurts cause of its back so yeh.Were gonna take her to the vet and get it checked out in a few days.Possibly tomorrow.If you have any ideas what might be wrong that be great it worry's me.If its a boy: im gonna name him Simon or Girl: Annabel. so please help me out? im 15 :P