She sensed something on your hand. It could have been moisture, temp change, something else you handled, or just something about your skin itself that got her curious so she decided to take a taste. Now sometimes it is a sign they are thirsty but not always. Some will only lightly give a tickle nibble, never breaking skin, but others will give it a solid chomp. I've noticed some species are more prone to this sort of behavior and within species certain individuals are more prone to it than others.
Usually you can tell they are thinking about it because their antennae will start to move up and down in a pulsating manner and then they will tilt their heads to look down before finally bending down. If I see one of mine is thinking about it I always just stick a little something not edible like bottom of my shirt or napkin, whatever is on hand, between where they are leaning over my skin. Usually they loose interest pretty quickly.