Hi Rachel!
There is a drop in sleep needs at about 18ish months, and there are a few threads ongoing with a whole lot of us in the same boat!
I am finding here the same thing, it's just reverse to yours. DS will still go down to bed, he'll just wake early. The past week or so I've had to limit to a 1.5 hour nap -- we do wake at 700, nap 12-130, bed 730. A few times this week I've let him sleep a bit longer for his nap, and although he'll still go down at 730ish, he wakes that much earlier in the morning.
It might be worth cutting her nap down by 15 minutes first, and see where that gets you, and then another 15 minutes if you have to. Rather than cutting it 30 minutes all at once, kwim?