Actually from everything I have read, premenopause can start as young as 35, and 38 seems to be a prime time for it.
Try Dr. Lee's book "What Your Doctor May Not Tell you About Premenopause" and Ann Louise Gittleman's book whose title I can't remember.
If your health is otherwise good and you are looking after yourself, then I agree with one poster here, suspect premenopause as the culprit. Nancy is lucky to have the hormones of a 30 year old, but perhaps she is also not the norm. Your lifestyle impacts your body, your hormoine levels. Nancy does not have your life, she does not have children, so her lifestyle impact upon her physiology will be way different from yours. And I am sorry but continued stress over aperiod of time does affect your hormone production.
Changing hormonal levels reduces the elasticity of the artery walls and begins symptoms of vascular irregularity, not serious, but irritating. Estrogen is responsible for maintaining the health of so many of women's bodily tissues and organs (see Dr. Lee), it is incredible. So fluctuations of this hormone will have tell tale effects. Premenopause may not be the "hot flashes" stereotype. Different women experience it in different ways. Only you know what's not normal for you. And if a short vacation doesn't cure it, it may well be more than regular stress and fatigue.
Dr. Lee's book will tell you what sort of tests you may wish to request.
Clare