Yep, your GP only prescribed half the treatment. If you have a womb and you take systemic oestrogen, then you have to take progesterone to stop the lining building up too much.
Normally they are very strict about it never being longer than three months without progesterone, maybe they meant for you to go back and get progesterone if you decided to stay on oestrogen?
When I started my latest HRT the nurse gave me the first 5-6 weeks with only oestrogen, then I had to start trying progesterones 12 days a month even though they make me feel very down.
I find utrogestan taken vaginally is the least bad one, I take that with Estradot50 patches, Evorel50 didn't work for me.