It's possible. I tend to not like "my lover betrayed me" plots though, because ugh. We've seen that before, right up there with dead wife stories. If Solas suddenly had a woman in his past who screwed him over (bonus points if he then killed her or watched her die!) I will take back every bit of praise I gave DAI for not having that old trope used for a love interest and kindly request that the writers get their collective heads out of their goddamn backsides. Enough is seriously enough.
I'd find it way more interesting if it was a friend, or the rest of his brethren. Someone important, but without the "mai troo luv" bull**** slapped across it.
I agree, but somehow I suspect it was his lover who betrayed him. I also suspect that she may not have been Ghilan'ain but some ordinary elf.
Regarding Ghilan'ain--we know she had some sort of tie to Andruil. She created the halla as a gift to Andruil, presumably for the sake of hunting halla. And there's some vague hint that halla were created from elven slaves (very vague, it's just a painting in red and black and white of elves inside of a deer-like creature--blood magic or sacrifice to create/alter a different existing creature, or to make elves into deer?). So it's plausible she and Andruil were lovers.
Fen'harel hunting the halla could have romantic connotations, as could the presence of his shrines in Ghilan'ain's Grove. But he is in the Crow Fens, and there is also a shrine to Fen'harel there. A shrine to Sylaise is not too far away on the map, either, but no one suggests Solas and Sylaise might have been together... That said? Fen'harel's "hunting the halla" might have been less romantic and more aggressive. If Ghilan'ain were the halla, and we're not talking about a literal halla (which he could have been hunting for any number of reasons including feeling it was kinder to kill elves transformed in such a way--if they were originally elves-- than to allow them to continue living as beasts), Solas may have felt she was dangerous or a threat to the People in some way. In that case, he may have been trying to capture or kill her.
As for other possible romantic entanglements, well, there is that one myth about Fen'harel being tied to a tree while Anaris and Andruil argued over him, and Andruil wanted to make him her lover for a year and a day. If Andruil wanted him and she was also courting Ghilan'ain, it's possible she was bisexual. It's also possible she simply wanted children, and didn't actually want Fen'harel for any other purpose. Other possibilities? It was a polyamorous relationship and Ghilan'ain, Andruil, and Fen'harel were all together with one another. Or, Andruil and Fen'harel were together and then Andruil left Solas for Ghilan'ain.
What we definitely do know is that Solas is deeply in love with a romanced Lavellan, and that appears to be an exclusive relationship. Therefore, his past romances shouldn't really matter that much to us. Maybe Lavellan means more to him than anyone else ever did. Maybe not. That doesn't matter, because what they had was real, as Solas himself declares.
There's also a good possibility that the romance is already doomed, and will never be picked back up. I know it's a negative thought, but in that case, does it matter if Solas had another lover, or is going to in the future? If we get DLC telling us it's truly over, then it's truly over. (Even though I hope there's some means to stay with him, somehow.) Lavellan's an amazing person, and she can totally find someone else if she wants to. If she doesn't want to, then she doesn't have to live her life moping about the loss of Solas. I mean, she has much bigger concerns than her love life.