Enemy shakes on Y-axis when is moving straight on the X-axis

First at all, sorry for my bad english.

Basically I have an enemy who moves toward player on Y-Axis and X-Axis.

When it moves in both, Y-Axis and X-Axis it works well, also, when it moves only in the Y-Axis, but when the enemy moves only in the X-axis it shakes on the Y-Axis a little bit but I don't know why.

Here's the code of the movement method.

 void Movement() {
     if (player)
         playerPosition = playerTransform.position;
     
     selfPosition = selfTransform.position;
     //Distance between player and enemy//
     distance = playerPosition - selfPosition;
     if (distance.x <= 0.0f) {
         selfBody.velocity = new Vector2(-enemyInformation.speed,selfBody.velocity.y);
         if (!isLookingLeft) {
             Flip();
         }

     }
     else { 
         selfBody.velocity = new Vector2(enemyInformation.speed, selfBody.velocity.y);
         if (isLookingLeft) {
             Flip();
         }
     }

     if (distance.y <= -0.5f) {    
         selfBody.velocity = new Vector2(selfBody.velocity.x, -enemyInformation.speed);
     }
     else {             
         selfBody.velocity = new Vector2(selfBody.velocity.x, enemyInformation.speed);

     }
 }