DS has been fine at nursery since he started at just over a year old. He does 2.5 days a week.
Aside from the usual initial settling in time, he adjusted well and seemed happy there. He started in the baby room then moved to another room at about 18mths and has been in the 2-5 room since he was 2. He moved up happily and was keen to be with the older kids. He would leave without tears almost always, until just before the last Christmas break when he became very tearful and drop-off, clinging onto us. This has continued and is getting worse if anything. It's really hard leaving him. Now, most days, he'll say not long after getting up in the morning 'I don't go to nursery today?' and if it's a day that he's due to go, and I say this, he says 'no, I dont want to go!' 
No idea why this has happened. There have been no big changes as far as I know. I thought it was caused by Christmas disruption that would soon settle once back in routine but it seems not.
Sometimes he settles v quickly after we leave but other times he takes longer and even his favourite staff or toys don't calm or distract him.
What else could be causing it and what should I be asking nursery?