Hi ! I do thank you for the replies , it is indeed as both of you said , I've waited on the results from the practice ( it always takes up longer to have the results of bloodsamples etc. here in Europe since it goes to the facculty and it is there somewhat ' busy ' ) .
Just have the conclusive report of the necropsy and bloodsamples that were send and she was indeed ' healthy ' but yeah she is gone right . So what caused her to die ? there is nothing other besides the fact her heart failed but no direct reason why this happened , bloodsteal got a list of some levels too high but not enough to say she would have died on that so that conclusion is simply striked out , she had no injuries nor was there any other organ failure altough he did notice something else too , she could have gotten some breathing issues right before she had her heart stopping yet no sickness that could have caused this , he thinks the change of the temperature outside that day so suddenly could be a reason , it changed in fact ' dramaticly ' that day to quite some good high temperatures while the days before the weather was still kinda grey / wet and far from that temp but no real cause of death in fact other then ' heartfailure ' . Now I really liked this girl in fact but I was given the tip to have a tub inside the pen with undeep water in it ( just rainwater would be enough ? ) because the weatherchanges in belgium are to what I believe quite known ? one day it is freezing in the middle of spring , one day the sun shines as if we are at the top of summer and according to him she might have been lethargic a little bit because I did notice that day they seemed to be more slow and even some very lazy , the amount of dustbathing really got up then lol but that is the tricky thing about it now , they seemed to enjoy the weather so I really did not see that one coming and it just had to be her argh ! she was one of the most kind and gentle birds actually plus I was finally begun with a ' project ' in sexlinked crosses wherefor I needed some eggs from here since there are always a few broody hens now lately while a time ago I did already lose some of my hens during an attack of a predatory animal ( one broody became cannibalistic and even ate her clutch and any other egg she could find from the rest so had to cull her ) now things were going good again and I'm having some changes too , like the older hens now ( used to have a quite big flock ) are now getting ' rid off ' ( rehomed a few and few are meant now for meatbird , some have aged over the 6y so i think it is fair ) , now I do personally believe that the changes with some new hens that are way much younger are going to be a part of the reason why she had her heart failing , perhaps it caused her some stress as i noticed she lost quite some feathers too before she died and I just assumed she started to molt but don't know about that one for sure ...
Just a small note too : usually in the heat of summer they have a tub , but since the summer didn't start really yet I had the tub this time not in the yard yet , gonna make work of that already to make sure if the weather suddenly becomes that warm again they can just get in the tub . Thanks again for the replies and I'm sorry too especially because she had such young age but it happened and like above is written it indeed can happen unfurtunately

, kind regards from out of Belgium