Goosieloosie, if you are a migraine sufferer, chances are, synthetic progestins will make them worse. Do you know why your doctor prescribed this type of HRT, was it because you don't want to have periods?
Your previous HRT contained norethisterone which is notorious for creating low mood, breast pain and migraines (although you say the migraines improved when you too Evorel?) but your new HRT contains the same progestin as the Mirena coil but obviously unlike the coil, it is not mainly concentrated in the womb and will circulate around your body.
The fact that your migraines have improved overall since taking HRT would indicate that you are the same as me, you need oestrogen to stop the migraines but the progestins ruin the good work that the oestrogen does.
Did you have migraines during your reproductive years and if so, where they just before a period?
After years of being on the wrong medication, doing my own research/trials and seeing Professor Studd, I now know that I must have a steady dose of oestrogen and a minimal amount of progesterone in order to avoid migraines - silent migraines in my case which consist of migraine aura without headache. I am not a migraine sufferer per se, I only get migraines if (a) my oestrogen levels fall too low and (b) I take synthetic progestins. I never had migraines during my reproductive years despite taking the BCP because apparently my own natural progesterone overrode the synthetic stuff. It was only when I stopped producing my own progesterone that I started getting the migraines when taking progestins. Synthetic progestins alter the brain receptors and cause migraines in some people.
It sounds as though you could be similar and if that is the case, you need to take a separate transdermal bio identical oestrogen (gel or patch) and bio identical Utrogestan (preferably vaginally) for 7 days per month. I now take 1 pump of gel every 12 hours (to get an even dose) and 100mg of Utrogestan for 7 days per month plus a pea sized measure of testosterone. It does mean having periods again but it is a small price to pay for getting back to normal. So far, it seems to be working!
Hope that helps.