DS2 is nearly 19 months old and has always been an excellent sleeper - 11/12 hours each night, waking only due to teeth or on one or two occasions a wet nappy having soaked his clothes; napping once for approx 1-2 hours each day for the last few months (down from 2 smaller naps).
However, literally in the last week, this has all gone to pot! He now refuses to nap in his cot. I can spend up to an hour trying to get him down & nothing works. He won't even sit in there for quiet time with a book (and he loves books!). He will still nap in the car/pushchair.
At night, we have a little routine - say goodnight to Daddy & DS1, teeth, milk from beaker, quick (5 seconds) snuggle, bed. He would lay in his cot, watch me walk off, snuggle down & go to sleep with 10 minutes max. Now, we do the same routine, but he has to be rocked to sleep in my arms, then I put him down & he wakes somewhat, so I have to stay & stroke his head & be visible to him until he is asleep. If he opens his eyes & I am not there he screams a horrible scream like he is being murdered!
He also now wakes at 5.30 each morning (used to be 7-7.30) & will not go back down, despite medicine (in case it's teeth), milk, nappy change, cuddles, books, toys in cot, music on...Again, the screaming is like nothing he's done before, it is like he's in real pain 
Developmentally, he has come on leaps & bounds over the past month or so. He learned to walk at 17 months & now rarely crawls & often runs. He has started feeding himself a lot (was very reluctant at this before) & just wants to spend all day pointing at things in books, pretend playing (puts phone to Mummy's ear, puts keys in door etc), climbing on the sofa to sit with us & copying big bro (had a massive tantrum the other day when I took away DS1's scooter which DS2 was trying to imitate DS1 on but couldn't reach the handles!) We get out every day to try & tire both DSes out!
I am going to take him to the doctors (very understanding & has said to me to bring either DS along if ever in any doubt at all) to rule out any infection as the screams really do sound like an animal in pain. But I think it is not that as he was loving fireworks last night (so obviously wasn't hurting his ears) & he is happy as anything once up & playing (mostly, anyway!)
Anyway, sorry for such an epic OP 
, but if anyone has any advice or similar experiences, I'd love to hear please!