Not a growth spurt. Babies go through growth spurts, in which they seem to eat all the time, and then slower periods of growth during which they do not seem to be as hungry. You might worry if your baby isn't feeding because she doesn't seem to feel good, is lethargic, or has fever or trouble breathing. If she looks and acts healthy, it may be a temporary slowdown in her growth. She may be eating more per feed, too.
Lotsa possibilities. Babies can go through periods of eating more or less depending upon growth needs. Illnesses often cause appetites to change, too. Family, social, emotional, and environmental situations have an impact as well. As babies get older, their feeding patterns often change, typically becoming stretched out a bit, though eating more per feed. Also, what they eat can alter their appetite.