I'm afraid I have no answers but my ds, also 19 months, is pretty much the same. He will often wake crying although there seems to be no physical reason for it and he too is otherwise happy in his cot. Sometimes he wakes up happy, which is so nice to see (and I have felt that this is maybe because he has had a decent amount of sleep as it is often after a good two hour nap). Actually, he often wakes after two hours, not necessarily crying but definately needing a few minutes to come to.
He has a long-standing habit of waking up to three times a night (though not all nights and not always as often) and will often let out a shriek and cry a bit as well. Often wondered why, and not come to any conclusion. Wondered whether he was having bad dreams. It has started to change a little as he is getting to grips with talking so sometimes will wake up saying 'cuddle' or 'milk' (meaning a suckle). Nights are generally disrupted to a certain extent and have felt that that is the explanation for his waking up with a cry or temporary grump. Once he has woken he can take a little while before he is his normal happy self getting on with playing etc, but just needs a bit of a cuddle or quiet, calm time.
Hope we get some more enlightening posts!! :lol: