I have bi polar type 2, and I took an SSRI once. if you dont know you have bi polar type 2and you take an ssri it will drive you into a manic episode. Manic episodes often can be characterized by hearing voices. I noticed these voices in my head as a distinctive third party, like a tv playing. It sounded like the typical horror movie "voices in head" scene, ten or twenty people talking all at once in a very ghostly manner, at first it was funny, then scary, then annoying and finally after a few hours it went away.
It seems both of you can distinctly notice the difference in your own thoughts and a "third party" thought. This can be caused by multiple personality disorders, OCD (I have this type of OCD, its obsessive thoughts, kinda weird concept but sometimes they sound like your own thoughts, but are very unwanted and "intrusive" thoughts that, although usually unplesant and petty, are often obsessed over, bi polar type 1 (manic episode) and multiple other things.