It sounds like an allergic reaction, at least it sounds like the kind of allergic reactions we've been having around here. Nicolas breaks out in welts when he comes in contact with dog saliva (our dogs can't keep their tongues to themselves!). He gets them in places that haven't had contact with the allergen though. He'll get them on his arms, face, etc. This also sounds like what I kept having pop up on my face in response to something (still don't know what, exactly, because it's seemingly so random). In our experience, if it's a reaction to something - like contact dermatitis - you'll see it again. As long as she's comfortable now, though, I'd just be glad it's gone!
ETA: just remembered Ella had a reaction like that last year at the beach. She had strange welts and raised spots pop up on her arms and legs; we gave her benedryl, it went away, then the next day it was back. This went on until we came back home so we figured it had something to do with the linens, but still don't know for sure.