I also wanted to say that my DD fed consistently for only 8 min from about 2 wks and has gained very well and very fast. (she was not premature, though). But just to say some young los can get enough in a short amount of time....do you have a quick letdown? ...espec if you bf before, you might have a quick letdown and she's not wasting time getting things going. My lo is getting milk within 30 seconds on the breast, so she can get a feed pretty efficiently.
DS was a different story, I often worried he was hungry because of a short feed or a lazy feed, and I got into that cycle of too many short feeds in a day. Once I stretched him out to a 2.5 to 3 hr EASY, I was a bit more confident he was not hungry, but needed sleep, comfort, or whatever. Once you start to see good gaining and gain a bit of confidence in her EASY, I think the fear will subside some. But as pp said, with her age/size/history, you'll want to be a bit careful and take that into consideration.