You say that she has children, so intimacy was not a problem in the past, with someone else.
Since you say that you were looking forward to having children together, your fiancee is till fertile, but could it be that she sees the menopause approaching and is frightened, of what it will "do" to her: reduce her attractiveness, cause her problems.
Reassure her that these are not real issues.
Maybe, she feels that having had children is enough, and does not want more?
Is there some health problem that thinks / knows she has but is not revealing to you?
You need to talk things through, to find out what happened six months ago to bring this on.
This is not a time to be anything other than extremely gentle and sympathetic.
Good Luck!
Alphabetical girls and boys names January 2024