If you have endometriosis (whether diagnosed or not) or adenomyosis, high estrogen can cause it to grow/develop. Since you just recently increased your estrogen, that would be my guess. (I have endo.)
The solution is more progesterone. Even though you are on the Mirena, you can have utrogestan as well. The thing with the Mirena is that its effect is localised to the uterus. Which is fine if you don't have uterine tissue outside your uterus, but if you have endo and have it elsewhere, then the Mirena may not help control that in the presence of high estrogen.
You need to try a systemic progestin or progesterone.
Utrogestan taken continuously at 200mg/night should do it, or you can just add in a POP like desogestrel - because you have the Mirena in the uterus so you've ticked the box for HRT progesterone requirements. Vaginal progesterone probably isn't going to work for the same reason - it's localised....
I'm on 3 pumps of gel now and just started taking 300mg of utrogestan sequentially day 15-26 of cycle. It seems to be working fine but, if it didn't, we'd move to continuous 200mg/night.