Posting as a teacher of 7 year olds rather than a mummy (my 2 are still small) but I hope I can be a little bit helpful: firstly if this is new then there will be a trigger, it may not be from home but from something she has heard or overheard at school in the playground etc, so I would check with the teacher and also ask the teacher if anything else has happened amongst her peer group (although she can't discuss individuals) she may offer some insight.
If this is not new but has suddenly increased and is now a part of her but suddenly peaking try not to worry in terms of development she is at the age where her understanding of death/mortality is being explored and although you have nt had a bereavement when you said 'I'm not going to tell her that won't happen' is good, there are good resources to share with your daughter even before a death has occurred. The book Badgers parting gifts is lovely and tackles death while looking at what gifts the badger passes on to his friends.
There is a really good book called a huge (think its huge could be large) bag of worries I have used with year 3 and it is really good at tackling what a worry is and how to handle them, which I would read with her.
Like someone else said the worry dolls are great and if she is into writing/notes etc (like many girls are) I would put a lovely journal/note book by her bed and explain she can write her thoughts down here, give her 5 mins before bed or in the morning where you put special music on/light a candle etc and allow her to write; this does 2 things a) she is releasing her worries b) you can use this technique to manage her anxiety a bit for example as you start to see her clam up start the anxiety then you can say something like; don't worry I can see you are getting stressed but you can write all about it at bedtime.
I recommend looking at her triggers if she is getting stressed at things being out of routine/change etc then release the tension for her by putting up visual timetables of the day or morning/evening routine etc
Last advice about looking at the cause would be to draw and label a map (birds eye view style works best) of your house, one of the school and any other significant places nanny's house etc and ask her to put a smiley face in the rooms she feels happy and a sad face where she feels sad and see if that helps at all.
Not sure the best way to words this but I know you are worried but once you have established no major triggers and shared with her some coping strategies read the worry book, I would give her quite short and almost neutral attention when she becomes anxious (I'm not saying she is attention seeking, but any behaviour is reinforced through our feedback) and when she is 'herself' lots of praise etc and hopefully it will pass.