Breast feeding

I could really use some advise. I'm starting to feel really bad and crying because it's not working anymore. My baby girl is 6 days old. She was born 2 weeks early weighing 6 lbs and I started breast feeding in the hospital. She did good until the second night home. She latched on but only sucked a few times and would let go then look for my nipple again and repeat this. She'd eventually stay on. It's gotten worse every day now. She latches for a few seconds and just cries latches and cries and so on until she falls asleep. I even had to pump and feed her with a bottle last night and one this afternoon because it had been too long since she ate. She also hasn't had a poopy diaper for 4 days until tonight but it was a very small amount and it was still the dark blackish/green color. I had a LC come out yesterday and she said I'm doing everything right and when she latched she was latching correctly. She's coming out again in the morning to weigh her and bring a nipple shield to try. Any help would be very appreciated! Thank you in advance!