If a child is simply gay, then there's nothing to be concerned about - of course. If she's having concerns about it or a hard time coming out, being accepted, etc. then being concerned, natural. My sibling and spouse had concerns for their child when they came out. Their child struggled with it.
So that was natural. Who wouldn't if your child was struggling? So is your child struggling? I guess you need to be involved with your child to see how they are doing - would be my answer.
I guess the part you seem to focus on is your daughter was heterosexual to begin with. I think that 'seems' to be the case for quite a few gay or bisexual people to begin with. They don't necessarily have it all figured out in the beginning. Sometimes they also don't feel that they have the support system in place to come out. There are a lot of reasons people don't come out at first. I guess that's another thing you can ask your daughter. These are all questions to ask her - or let her tell you, when she's comfortable. Just be there for her. I wouldn't press for answers at this point.
Debra made a good point for kids at the middle school/high school level. These days, being bisexual can be seen as a 'fad' for some kids. It has been for some of the kids my teens know. They know friends (more acquaintances in their larger social sphere) who 'become' bisexual for a few months. There's even clubs at their schools - I think rather informal, but that have names, and kids join them. I don't fully understand, but my kids don't really think anything of it. Kids seem to experiment more these days.
I do know a few instances where girls had some bad experiences with high school boys or early college, and turned instead to dating women. In that instance, I would be concerned. They later dated men again - eventually. That's different - again, I think than being necessarily gay. I also knew a teen/young woman who dated women when her mother went through a series of relationships with multiple men (was married several times). Again, I think she just swore off men for a while. Some would even say she was attention seeking (needed her mother to be involved and instead found affection from other women). I have no idea - not a therapist.
I guess so long as your daughter is emotionally well and stable otherwise - and is happy in her relationship - what's there to be concerned about? I would just remain close to her, and make sure she can come to you :)