No she is not necessarily a lesbian, I think it is possible for a woman to connect with another woman and for that relationship to develop but this does not mean that she is eyeing up all women. Perhaps there is just some special bond she has found in this person.
If she has been honest enough to admit that she is in a lesbian relationship then I would assume she is being honest in everything else she says, however you were engaged to her so you know her best.
As for getting back together, maybe this is a blip and she is exploring her sexuality, just because she is in her mid twenties does not mean she is too late to experiment or question her sexuality.
Take care
warpig