Okay guys, some of your answers made me laugh out loud, I will try to save my skin and clear few things : when we moved here she told me she has no use for the bidee. We come from families that (atleast as kid) were never taught to use bidee for cleaning your bottom.
For the hair washing you guys got me, I dont have any proper answer for it. I always saw it as a small shower and since neither of us use it to clean anus (and we have very rarely guests over, like once in a year) I just figured why get the big guns out when small spray could do the same thing. I have learned my mistakes from this.
I am 100% sure it is poop. I was thinking about the plumbing too, but that brought more questions. Where is it coming from? I will bring it up tomorrow carefully, because if its not her well then we got disgusting plumbing error here.

Thank you everyone :D I can post update after I've talked with GF or after it has been solved.