Rhetorical question

I wrote this message in another section, so I'll just rewrite it here, for I think it fits better in this section.

I've been hearing lately expresisons such as "I don't want that... Or do I?" or "I am so mad... Or am I?".
This seems to confuse me, since it looks like the first part of the sentence can be both in affirmative or negative form.
Are both correct?
thanks in advance.