IMessage problems. Help please?

Me and my friends both have iPhones and text each other a ton of times using iMessage. But today all my messages turn green meaning their sent as normal texts. This would be normal if she'd be replying to the messages I sent. It's so odd I feel as if she's not getting my messages. I know all my settings are correct but are there any reasons regarding her phone as to why the messages are acting weird like could it be her phone is dead or off or just messed up. I'm just worried about it cause things been kinda weird lately