Must be attrition. Highlight all your armies and check to see if any of them have anything other than a green 0% next to their summary on the left of the screen. 0% means no attrition. 10% means 10% of the troops are lost each month and are then replenished from your manpower pool. If the number is red or orange then they are not able to reinforce fully but regardless, anything other than 0% means you are bleeding manpower. During peacetime this is completly unneccesary. You're losing a lot of men so check for armies in boats, unclaimed land and other countries' provinces.