8 mo won't nap

Anyone else having trouble with naps all of a sudden? You'd think I was torturing my baby the way she screams when I put her down for a nap. Before, I read her a book and put her down awake and she'd babble herself to sleep but these last few days she is sitting up just crying. How long do you keep your baby up between naps? Maybe I'm waiting too long? I go off sleep signs and it's usually between 2-3 hours of awake time. I don't know what to do!!!