my almost 3 wk old slept too long

So you would think I would be excited that my baby (will be 3 weeks old on Wednesday) just slept from 12-30 to 5 a.m- but in freaking out bc he should be eating every 2-3 hours bc he needs to be back to his birth weight by his appt on Wednesday! He hasn't gained it back at his 2 week visit (still had 6 oz to go). He usually wakes up at least every 3 hours but he slept right through so of course I did too. He is nursing right now. I feel so guilty he didn't eat sooner. Is missing one feeding going to make a huge difference or am I worried for nothing?