Me No Sleep

So out of nowhere, Isabelle decides that she doesn't want to sleep anymore. The last few nights have been a little rough. She took a short nap last night after putting her to bed much later than we normally do and then was up screaming her head off for at least an hour after that. No matter what we tried, she just did NOT want to go to sleep. She was just laying by her bedroom door if you went in there. Then this morning when I laid her down, she's been playing her room and banging on the door. How do people with multiple kids do this? I mean, they're directly across the hall from each other, so I guess they'll both learn that they are SUPPOSED to keep sleeping even when the other is screaming. Argh, She's been an AWESOME sleeper up to this point and I hope with all my heart it's just a phase.