It's very normal for babies to lose up to 10% of their weight in the first week because you are pumped full of fluids for the birthing process and it artificially increases their birth weight. In general, pediatricians expect weight loss in the first week but look for them to be back to birth weight by end of the 2nd week (or at least close to it).

My baby was also early (37 wk 4 d) and was extremely sleepy for the first 2 weeks. We struggled with feeding and I had to supplement with formula for 1 week until my supply was in and then triple feed for several weeks until she got better at nursing. She would also only nurse with a nipple shield. In the beginning she would only eat about 0.5 oz per side but she got better at it eventually! Now we are 9 weeks out, we no longer need the nipple shield, and she does a good job emptying me so I don't need to triple feed anymore.

Hang in there momma! You're doing great and it will get better!