Why do you go back to sleep???!! Join in darn it!! Maybe she wants more than one helping a day...trust me, I've been in her shoes!! But if she starts pushing you away, then there might be a problem.
The only way to know whats going on is to talk to her. Ask her if she feels shes not getting enough sex, ask her if you can join in....heck ask her if she wants a dildo!!
I'm sorry hun, but sometimes women like to explore themselves, maybe shes thinking about an earlier sack session and she wants more!!
If you can't perform, ask her if you can masturbate her instead!! Lend a "helping hand" **giggle**
But all in all, I wouldn't worry about it at all, it's normal. If you let it eat you up, you might end up hurting yourself in the end. like I said, as long as she isn't refusing sex or pushing you away or avoiding you, then there's nothing to worry about....she's just hungry for seconds