First of all, check latching on position. Hold her at right angles to your breast and make sure the whole of the pigmented bit round your nipple is in her mouth. If her nose is buried in your breast, just put a finger gently on your breast to keep her nose clear. Purists say you don't have to do this if position is correct, but I had to do it, and I successfully breastfed for 20 months and 22 months. However, after the first 4 to 6 weeks, there will be more space between nose and mouth and you won't have to do that.
She should suck vigorously at first, and then change to a slower, rhythmic sucking. If you notice this with most feeds, then the letdown reflex has kicked in, giving her the high calorie milk which comes after the watery milk to quench her thirst. If you never notice it, then she's probably only getting foremilk. Possible reasons are incorrect latching on, or tongue tied. It's a very good sign that her nappies are wet and dirty. I don't think she's losing too much sith vomiting, since she's giving you these nappies.
Frequent short feeds are common, and don't indicate a problem. Weight loss is concerning. Hopefully she'll start gaining weight soon. Good luck. Hope it works for you, but if it doesn't, remember you've already given her a good start with the breastmilk she's had, and the colostrum. Enjoy your baby, however you feed her. One way or the other, things will get better xxx