This stage brings with it self-assertion, testing and protesting. It's no wonder that sleep often
goes downhill during this age (not to mention behavior). Kids start to resist more and parents start to give in. And then a tiredness cycle sets in and things just keep getting tougher all round.
This stage will likely be tough regardless (especially if you have closely spaced children), but don't let tiredness make it tougher. Don't let your child's protests convince you that she doesn't need a nap or an early bedtime anymore. She does. I promise!
Her sudden pouting when you put her to sleep (after she's gone to sleep happily for months, may I add) likely isn't because something is wrong (but you will want to check to be sure, of course). She is probably testing out this new power of hers. And she is testing it out on you!
She may also take longer than usual to fall asleep and, well, she may do all kinds of weird things with her sleep. It may be related to changes that need to be made (like an increase in waketime or dropping the 2nd nap), but it may be something that will only go away with time.
So how in the world do you know what is causing what?
That is hard to say for sure (but there are lots of posts on that to help you out!). Knowing the averages with other babies helps. For example, when they normally drop a nap or their waketimes or their total hours of sleep. But sometimes you just have to try something out to see if it helps. You know, increase that waketime a bit and see if your baby stops playing for 45 minutes before her nap. Or maybe you will decrease nap time a bit to see if it helps with night sleep.
Whatever happens, try to be consistent with your methods. Avoid making new changes if possible. And show lots of love but make sure to be firm and let your child know that you are the parent.