Feeding at night

Baby is 12 days old. This evening she was eating a lot - like once every 1.5 hours or less, for 30 minutes at a time, and I gave her some formula because I was worried she wasn't getting enough milk. After that she had a couple more short feedings. She has now been asleep for over 3.5 hours and will not wake up to eat. She is not unresponsive, just doesn't seem to want to eat right now... She actually seems frustrated I am even trying to feed her. I have no idea if she is back up to birth weight because her next appointment is Friday. Has anyone experienced this where baby cluster feeds and then doesn't want to eat at night? Is it safe to let her sleep longer than 4 hours? 😥