Hi Sammiejane,
I think one possible explanation is that the oestrogen you had been taking is being opposed by progesterone in the joint tissues once you've started taking Utrogestan, the interplay with receptors and all that complex stuff. When you start taking exogenous hormones, the whole body has to adjust and it takes some time. All I can say from personal experience is that fluctuating hormones in perimenopause were the cause of my joint pains. Rheumatologist said I have osteoarthritis and prescribed a lot of drugs, but I refused them. I have endured the perimenopause to menopause transition with joint pains, but now they're gone.
BeaR.