My DD was the same way, her skin was so sensitive, it would bleed when I wiped it soometimes. The best recomendation: no diaper as much as possible and to try to let the area completely dry btween changes (fan it if necessary, they love the feeling anyay! lol) It may be the causes you suggested but realize the more she scratches it, esp. if a rash pre-exists) the itchier it gets.
BTW, I have a DS now who has completely diffferent skin. 14 months, never had a rash at all either.