Well, it depends on which theory of time travel is correct.
If we go with the most popular, being "multiverse theory", if you fuck yourself in the ass in the past, the prime timeline would shift, causing what was an alternative timeline in the multiverse to now become prime (the one where you were fucked in the ass by yourself), and what was the prime timeline to now become an alternative (the original timeline where you didn't fuck yourself in the ass in the past). The transition would generally be imperceptible to people since such a tiny shift would most likely ultimately be fairly inconsequential to the timeline. Of course, it is also possible that doing so creates a "probability cascade" that leads to a "fallout event". This is...