it actually doesn't have to be something you've changed recently that could be causing this. my daughter broke out into horrible hives/welts in reaction to some strawberry bubble bath that we'd been using for at least 6 months! our doctor said sometimes it just takes that long for an allergy to develop.
it could also be something in the air she might be reacting to..... if there are any lawn companies around the neighborhood spraying fertilizer on the grass (painting it green), or bug sprayers, or anything else in the air (pollen, grass, etc....).
unfortunately it's might have to be a time of trial and error until you figure it out. i'd keep a log of her daily routine and activities.... keep track of when the hives seem better/worse and watch how they correlate with what she's doing/eating each day. hope this helps some!