Heaven I went through a similar thing with DD at around the same age.... it happened during transitions from 3 to 2 and from 2 to 1 naps. Sometimes even now she will still have two naps, but if she wakes up from the last one after about 3.30 we're in for a long night. She was the same in that she would.not.sleep unless she was awake for 3+ hours first (ETA now it's 4+ hours ). Can you try making sure she is awake after her afternoon nap at about 4, even if that means she only sleeps for half an hour? it's a pain I know but sometimes it's better than her fighting you at bedtime.