What is happening to my perfect sleeper?!

uptonogood

DD has had a bed time of 7-7:30 since she was around 4 months old. She passes out and is out for 11-12 hours. She's done this every night for MONTHS! Well, the last two nights, she isn't ready for bed until almost 9 and when I put her down she just screams and cries. She NEVER cries at bed/nap time. And at nap time today, she fought it like crazy for 45 minutes before I finally put her in her RNP to nap. Any idea on what her deal could be? I don't think it's teething because she's never acted like this before (she has 6 teeth). I'm at a loss. I just feel so bad because she never cries. She only cries when something is really, really wrong.