2 week old feeding properly?

My baby girl is 2 weeks old and I’m worried that she’s not getting enough milk? She latches fine and frantically sucks for 3-5 minutes, slows down and falls asleep. I do a nappy change and she will wake up, I put her on the other boob and she will barely suck for 3 minutes before falling asleep again. She does go “floppy” after and has lots of wet and poopy nappies. She’s was 8.2 pounds at birth and is now 8.7 pounds. She only demands food every 4-6 hours but only less then 10 minute feeds. Surely that’s not long enough to get a full belly?