If you are saying that you were doing the exact same thing at 13 as now, no changes in techniques, fantasies, or situation, and the amount of natural lubrication has changed, maybe your body just doesn't find it as exciting, or maybe it's just been a different time of the month. (Your natural secretions are much thicker at mid-cycle than they are right after your period or right before the next one.) I would try not to be watching and judging yourself. That can really get in the way of sexual excitement. Just go with what feels good and don't worry about quantity of wetness.