What happened to my beautiful sleeper?

Since Lily was 4 days old she has sleep constantly through the night(we're talking 10+ hours here).
She got her first big bed about 4 weeks ago. She went into it straight away, no problems. I felt she was even sleeping better and not getting up tired.
So why in the last few days does she cry when we put her to bed? She is dead tired, eyes hanging out of her head, yet she just cries for either DH or myself.
So I go in there because i hate seeing/hearing her so upset. She is happy and i will sit with her for a few minutes. She will play around, have a chat, etc. I ask if she is tired, she says yes. I ask if she wants to sleep, she says no. So i let a few more minutes pass and she seems happy. So i lay her down and put her covers on, she gets a little whingy. I leave and the crying starts again. So i have to go through the whole process again. I don't mind this, however, i just would like to know WHY she is upset about going to sleep, when the last almost 3 years she has been a perfect sleeper?

Hmm.... the age Lily is at can actually be a really tough age for sleeping problems I have found.. Aidyn has had the majority of his sleep problems since after the age of 2.5, up until very recently.
Perhaps maybe the new bed actually has unsettled her somewhat, even though this seems to have taken some time to kick in.
Lily is also at the age where her imagination is kicking in, so she might have an irrational fear of something that you are not aware of? I think this is a pretty common reason for toddler of that age to be upset at bedtimes...

Recently we gave Aidyn a little lamp on his bedside table, right next to his bed... and the difference it made was dramatic (it was also combined with giving him his dummy back for bedtimes). Now when he wakes during the night he isn't upset and crying at all, so I actually think its the light that is stopping him from instantly being scared.