Hi Eleni!
This is the way my gyn explained it to me: when you're younger your estrogen is way, way high, above average. When you start to have perimenopausal symptoms your estrogen is dropping even tho it is still "within normal limits", thus explaining the way you feel. I went through 2 family practice doctors before being referred to my current gyn and finding this out. They kept telling me my tests were normal and one almost had me believing that this was all psychological! (Can't wait till she starts going through this!) The book I recommended below is also helpful.
Take care!

Cat