A baby that young needs to eat every 3 hours and you start counting from the beginning of a feed. That means if the baby wakes up at noon to eat, the next time she should eat is at three o'clock, even if it takes her a full hour to eat due to trying to keep her awake.
If she wants to sleep, that's fine. Some babies that young need and want to sleep. My ds slept all the time for the first few weeks! But they do need to be awakened to eat. So let her sleep and wake her every three hours. After feeding her, change her diaper. If she can stay awake 10-15 minutes longer for activity time, that's great. If not, just let her sleep and start it over again 3 hours later.
Make sure to mention this to your doctor, though, just so she knows how much she's sleeping.