Same question here! I don't really understand the ending of DAI...at first I thought she took Solas' body instead,and then the Trespasser DLC came out,Solas is still Solas...so,if Solas is actually Fen'Haral,and Flemeth is actually Mythal,what's their relationship? Why Flemeth chose to petrify herself infront of Solas?....guess we'll just have to wait and see in the new game.
HUGE DAI AND TRESPASSER SPOILERS, YOU HAVE BEEN WARNED...
I watched it again and my interpretation is that Solas killed her or took Mythal from her (as I don't think she's Mythal, perhaps she just nicked her soul?). If you watch Solas' face just before the magic happens, his expression changes, therefore he did something to her. I think. No doubt Bioware will prove me totally wrong. 
I guess the other option is she infected him so she could control him, but after Trespasser that seems less likely to me. And also I hope not, I'd prefer it the other way round, that all this veil **** is about to go down because Solas has reasons, not that Flemeth wants to take over the world by using his body.