DaisyB, I would put money on your problems being related to the Mirena coil coupled with very low oestrogen.
I have had two Mirena coils, one pre-menopause and one post menopause and I can tell you from personal experience, low oestrogen combined with a Mirena coil is bad, bad news and should be avoided. In theory, the progesterone in the Mirena coil is confined to the womb but it is not and enough of it will get into your overall system to cause side effects and undermine the good work the oestrogen is doing.
In my case, once my oestrogen levels dropped too low, I started getting silent migraines which I didn't get with the Mirena pre-menopause and looking back, yes, I had nausea too.
I used oestrogen patches with the Mirena and they were completely useless and didn't provide anything like enough oestrogen - this was confirmed in blood tests. I even worked up to a 100mcg patch and still it was no good. I have since had the coil removed and now use Oestrogel and Utrogestan but with the benefit of hindsight, I wish I had tried Oestrogel with the Mirena because I think it would have worked much better. Firstly, I absorb the gel extremely well and get more oestrogen into my system and secondly, the gel is more flexible than patches and it is much easier to quickly adjust the dose if you need to. I didn't like wearing patches either but that was a personal thing.
My conclusion is that the dose of progesterone in the Mirena coil is too high for a lot of women meaning you need a high dose of oestrogen to counter the side effects. I found that the only time I felt good was when the coil was running out of steam towards the end of its life.
My advice would be to contact your private specialist and ask to have the oestrogen prescription sooner rather than later and I would also ask for Oestrogel. In the meantime, I would definitely stick a patch (or two) on - if you start to get any lining build up, the progesterone in the Mirena will make it shed.
I hope that helps and as Jenna says, you might want to read my old posts. I can't guarantee you will react to each product in the same way but you have nothing to lose by trying.