DS was bf til 7mo and has always been a great sleeper/napper (when he was bf he would wake up every 2-3 hours at night but gradually grew out of that as he got bigger). When he went on to formula he learnt to get himself to sleep so after bedtime routine I would place him in cot and he would just drift off in his own time.
This was all great until about 2 weeks ago when he started waking up and screaming in the night several times. After I pick him up he falls asleep straight away on my shoulder and I'll sit in the rocking chair for a few minutes with him (he's usually gets himself into a right state after crying even though I go straight to him) until he calms down. He will be fast asleep and snoring but as soon as I put him down he does his almighty loudest possible scream until I pick him up again!
I just don't get it, he's not clingy AT ALL, I can go out for the day and he would never realise and isn't bothered if I leave the room etc during the day.
He seems to be teething a bit so is having neurofen which seems to make him sleep longer but if he does wake up at night it's still the same screaming routine. I can spend up to 2 hours picking up and putting down until he's so tired he can't cry anymore 
I just can't figure it out?