My two DSs were exactly the same and just like you I was worried. As long as she is gaining weight and has plenty of wet nappies I wouldn't be waking her unless she seems really drowsy during the day too. My DSs were quite alert when they were actually awake so Id say that would also be an indicator as to whether she is getting enough nourishment. As long as everything is ok I say enjoy the sleep while you can get it because her sleeping could change again at any time.