She does not have to take a nap at the same time she has always. If it takes a longer while for her to get to sleep, then she is probably not tired yet. She is older now. And kids internal clocks change!!!
People who have long term set naptimes, do it for their own motivations. Not their childrens real needs. Out of convienience for you, or if you believe it helps her, then continue. But be open to be flexible. My 6 year old quit taking naps after 12 months old. Not because I wanted him to. And he would get a little testy late in the evening, but strong willed, refused to nap. Who cares! I could count on him to fall asleep like clockwork at 8pm, sleep solid for 12 hours (ok for that young age) and wake up at 8am. Yes it was odd, but consistent. My 2nd child, a 4 year old falls apart, sobbing at around 4pm. (too late for a nap/too early for bedtime) So fine, I let him nap naturally, when he needs it, but limit it to a solid R.E.M. 90 minute increment or so, wake him at 5:30 or 6pm, and make sure he plays hard for at least 2 or 3 more hours, and he also sleeps through the night beautifully. My 3rd child, a 2 year old is in a developmental transition, which happens, and I will let it. He may or may not take a nap. He may take 2 long naps, 1 short, but he will balance out again, and show me his pattern soon. My 4th child, who is 11 months old, wakes up, and like clockwork is tired 4 hours later. He then takes a 1 1/2 hour nap naturally, and lately has sporatically missed his second daily nap, because he is changing again. (getting closer to the 1 middle of day nap) Sorry my reply is long, I think my suggestion is, to stop, be flexible and let her go a day or so on her own, without initiating her nap. Too see what her bodys (cant spell) natural needs are. You never know. But she will let you know. And obviously she is letting you know of her need change now.