Try selecting do clipping in the mirror modifier. Or just try rotating it around, it doesn’t really look like you have it applied unless you made both sides editable. Maybe upload the .blend file?
j1adobe,
the only way I can reproduce your result, is by rotating the half to be mirror before mirroring (M,X) it; and/or using a misplaced pivot centre. Alternatively, to use a misplaced object as mirror object. As far as I know, it is not possible to achieve your result using the cardinal coordinate axes as mirroring directions.
/ Mats
Note: Judging from your screen shot above, you are not using a mirror modifier. What to use is of course depending on what you want to achieve, by in most cases a mirror modifier is what you use for symmetric objects.