Context is rearing its head here.

In "Really, Monica is not nice," Really might mean Actually or I insist, believe me.

In "Monica really is not nice," it probably means actually.

In "Monica is really not nice," it might mean actually or extremely.

In "Monica is not really nice," it might mean very or actually.

In "Monica is is not nice, really," it probably means actually.

In "Monica is not nice. Really," it probably means I insist, believe me.