Ok, she appears to be pregnant. If so, her nutritional requirements went up because of the calf she is carrying. Are you feeding her once a day or twice a day? .5 acres of pasture is not enough to really make much difference one way or the other, so I would discount it entirely and chalk it up to exercise. Increase her hay to a 65lb bale every day and a half, and feed her twice a day. It sounds like she is simply hungry. You might also consider putting out some mineral for her, because it is an essential need for cattle.