Okay, I'll keep this question plain and simple. Whenever I have cube use a subdivision surface and attempt to extrude vertices with that cube I get really awkward results because whenever it adds a vertex the model gets an awkward dent in it as seen below.
When you extruded, as you didn't activate the Clipping option of the Mirror modifier, you created a inner face on the mirror axis.
So you can either:
Cancel your extrusion and redo it with the Clipping option enabled
Or delete the inner face: On the bottom of your 3D view, click Face Select button, disable the Limit Selection to Visible button, you'll see the inside face, then select it and X > Delete Faces.
Also, you should put the Mirror modifier above the Subsurface modifier.