Sometimes babies just have sleepy days. Trust your instincts. If you think somethings wrong go to the docs or you could try calling 13 health, they have been very helpful for me. If nothing else it wil give you peace of mind.

Overtired is definately a possibility. You could try giving her a dream feed if she'll take it, I dont know if that will work given that she's so young, but could be worth a shot.

If she's happy and alert during her awake time I'd think she's just catching up on sleep or going through a growth spurt.

HTH