Oh my gosh!! I have a 7 week old and it sounds exactly like her!!
We've only had her home a week (she's been in NICU and SCN) and she slept beautifully for a couple nights.
Last night she didn't go to sleep til 1.30am. Up at 4. Grizzled til after 5. Up at 6.45 to start the day.
She likes to stay awake for her nighttime feed around 3am.

Frustrating!