My 18 month old dgd has suddenly started behaving rather like the OP's. The only real difference is that she has never been a consistently good sleeper, despite all DS and DiL having a good bedtime routine.
This last couple of weeks, however she has started resisting sleep at any time (including daytime naps at my house, not just when she is at home), despite being obviously tired.
At night, as well crying at bedtime, she has started screaming in the middle of the night, and the only way of calming her has been for her parents to take her into their own bed, where she snuggles up to them, calms down and falls asleep. They are aware that this isn't a good thing to be doing on a regular basis, but are so tired from lack of sleep that they are then struggling at work next day, and this at least gets her back to sleep.
Monica Because she's so young, the possibility of night terrors hadn't occurred to either me or, as far as I know, her parents, but your mention of them has set me wondering if she might have started having these. I will mention it to her parents, to see what they think.