It's possible that she is hitting a growth spurt and by this age newborns are generally more alert and awake and they will let you know it! So she could just be a bit miffed that your let down could be a bit slower until it catches up with her. When you lean back to feed, it can slow down your letdown - this is a common trick that Mums try who have a fast let down to slow it down a little so if try to avoid leaning back while you feed it could help