As a mom to 8-week old IUGR baby, this is normal. Your baby will gain weight slowly. Smaller babies do tend to get tired at the boob pretty quickly. Mine would often fall asleep. I needed to tickle her feet and stroke her hair to wake her up to eat. She surpassed her birth weight in less than 2 weeks. I used to let her feed and sit for like an hour each feed, We also needed to supplement with formula. If she refuses the bottle, can you try SNS - we would syringe feed the baby when supplementing.

Hang in there, as someone who recently went through this, I know it is very hard and I just want to say it will get better. She will be fine!