You do get random sudden deaths in younger birds of any breed in my experience. We had a Wyandotte die for no obvious reason at about laying age -came out of the coop and was dead an hour later. Same with two Cream Legbars. May be something to do with hormonal changes, may be a heart attack (purple comb). I remember reading something about a so-called 'lethal' or 'death gene', but can't remember any details.
It could be red mite WoodlandsDevon. If she has a pale comb and they are really bad. In which case you will lose more soon. As I'm sure you know red mite multiply fast so need more feeding -depending on temperature they can double in a week. Had a lot of soft shelled eggs recently, or eggs dropped off perches?