Most babies go through this right around her age. It's completely normal and usually called a "nursing strike". You can probably find more info on www.kellymom.com about how to deal with it, but if I were you, I would put most emphasis on nursing, and less on formula and food. Nurse her first, THEN feed her food, so she gets mostly filled up on breastmilk (which is best for her right now anyway). Food before a year old is more experimental than anything, breastmilk should really be her biggest source of calories (and healthy fat). The more she fills up on food, the less she will nurse, and the more formula she gets, the less milk you will produce for her.
My son has been tiny, and we always worried about his weight. For a while, he wasn't even on the growth chart. If I had to do it again, I would have nursed him more, and started food much later.
Good luck to you!