You may not have closed the lid tightly enough.
Without the cap, your bottle will lose a little water when you tip it. This is due to the built-in pressure equalization valve - the small gap at the top between the mouthpiece and the body of the bottle. To make sure your bottle is completely sealed, you need to put a Pod on the mouthpiece and screw the cap on. As soon as the Pod and the cap are firmly on the bottle, all air holes are closed by the automatic deactivation of the pod. This means that no more water can escape.