Is this a sleep regression (16 months), and how should I handle it?

[nutmegandginger]

My DD finally became a good sleeper around Christmas (then aged 13 months), when she started sleeping through 12 hours without wanting to feed. We didn't do any formal sleep training, just I didn't go to her in the night when she woke but someone else did and offered her cuddles and a drink from her cup instead (which she refused). After a couple of days, with virtually no crying, she seemed to stop waking up altogether. Problem solved, we thought...

But in the last few weeks (now aged nearly 17 months) she's gone back to waking at about 2am, and wanting to breastfeed. If DH goes to offer her water or a cuddle, she pushes the cup away angrily and won't resettle, but just cries and points at the door or says 'mama'. He's tried this for several nights now, but he hasn't been able to get her to stop crying without bringing her through to our bedroom (whereupon she starts laughing as soon as she sees me). When I feed her, she nurses quite enthusiastically and then goes back to sleep straight away.

I'm posting to seek out opinions about how to handle this. On the one hand, I don't want to create a habit, having only recently established night weaning (I had thought), and so part of me thinks that we ought to be a bit tougher with her, and tell her no milk till morning, since she was able to sleep through with no issues until quite recently. Having undisturbed nights was so brilliant that the idea of backsliding really worries me. But on the other hand, I'm wondering what if it's a short term phase, and she'll go back to sleeping through naturally again when it's over. If it's just a short term thing caused by some kind of developmental issue, I hate the thought of letting her cry. She is just learning to walk, and going through a massive leap in terms of her language right now, and I think she is also teething so I understand that she has a lot going on.

In case it's relevant, she goes to bed happily at night now with no fuss. She has one daytime nap, the length of which varies, but usually between 1-2 hours (night time doesn't seem to be affected by how long the daytime nap is). She is eating well (having been a rubbish eater in the past), and so I'm finding it hard to believe she's really hungry at night - it's possible she's thirsty, but she is so furious at the idea of water from her sippy cup that it's hard to know.