I'm thinking still might be a little bit young to be frightened of the dark yet....we're only just starting to go through that now with our 2.5 year old...but I guess it's not entirely out of the question?

I'm thinking the more likely reason would be separation anxiety at her age...my DD is also a textbook sleeper, has been from birth but I recall we went through this with her between 10 and 12 months.
I was a phase for her and she did outgrow it, basically we just kept her bedtime routine consistent, kept reassuring her, and rode it out.
If your little one has always been a good sleeper it's not likely to all go to hell permanently at her age, but she'll have phases like this from time to time. Hang in there I'm sure she'll be better soon.