P.G.
Hi Bethany - call a lactation consultant as soon as you can to help with the nursing. You may need a stronger pump (hospital grade) as NOTHING can express milk like a baby, and store-grade pumps aren't as strong. Your baby may have a problem with sucking you can't figure out, and a professional might be able to help. Ask them about your weight loss and whatever other symptoms you have, just in case they've had a similar case before.
Also, I'd go to an urgent care clinic and call your doctor for yourself. I don't know how you're feeling in general, but it sounds like your weight loss is making you nervous. You need to take care of yourself so you can take care of your baby.
I hope it all works out! Keep pumping and encouraging her to nurse, even just to suckle if she'll do it.