At 18 months it could be 2 things: either nap time needs to be later, as you suggest, or canines are coming in making her more tired than usual so nap needs to be earlier.
You know your kid, so if teeth tend to make her sleepy than you might try a little earlier nap and see how that works. However, even if most teeth didn't seem to bother her for some reasons the canines are the King Hell Teeth so it's still possible that they are making her OT even if teething doesn't usually give her that reaction.
If she's always had a longer A time than most kids, then go straight to moving nap later. Either way, you'll probably have to experiment a bit! I guess start with later nap time first; if that doesn't work after 3 days then give the earlier nap time a try. If nighttime sleep also starts to fall apart then I think the teeth/OT scenario is most likely.
Good luck! For some reason 18 months is one of those ages like 3 months where pretty much every kid goes thru a weird sleep blip! I blame it on those evil teeth!
