DS3 (who will be 3 next week) has, bar a little blip when he was a baby and the odd period of 5a.m. starts, been a good sleeper. Until recently, he used to go into his cot, tuck himself up on his tummy and fall asleep pretty much straightaway, sleeping through until about 7.
A couple of weeks ago, DH and I went out for a rare night out, and a friend (whom the DC know) babysat. DS3 was still awake when we were getting ready to leave, and had a meltdown - "No, Mummy, don't go out!" He fell asleep after a short while, but woke later in the evening, howling, and was up in the night, also howling, although he did go back to bed.
Since then, his sleep has got progressively worse - he will no longer fall asleep on his own, seems scared of the idea of going to bed, howls and cries for ages (he is up there now, hollering - DH is with him) and is then up multiple times in the night, often screaming the house down when we try to put him back to bed. I have resorted to sleeping in with him, as he will then sometimes go back to sleep.
He is having a lot less sleep than usual, so is cranky and over-tired. As he kept climbing out of the cot, and we thought that this might have a novelty value, we've moved him to a toddler bed. He is also not napping much, whereas previously he would happily have a 2-hour nap, although we were starting to worry that this was too much and he would not be tired at bedtime.
Is it just too much change all at once? Have I traumatised him by trying to have a life? Do toddlers sometimes just go through phases like this? It is exhausting, frustrating - but at the same time, I feel sorry for him as he seems so unhappy and tired 
Any ideas??