The question you ask has been asked and answered before.  As you point out, your condom almost certainly came off upon withdrawal from your partner, not during sex.  If it had come off during sex, as you point out, it would have been more challenging to remove.  When a condom comes off during withdrawal from a partner, the condom has typically done its job, protecting both partners from risk for HIV and other STIs.  This is the case irrespective of the position that sex occurs on.

Further, I would point out that if your partner did not have HIV six months ago it is quite unlikely she acquired it in the past six months.  Whatever she was doing to avoid acquiring HIV before had worked until then.

If I were you, I would not pursue testing related to the event you describe.  Obviously however, testing is a personal choice.  I will add that I note that this is your 6th question in the past 6 months, most of which deal with very low risk circumstances.  While this question is on a different topic, like your other posts, it does deal with low risk situations and it is for that reason that I chose to answer it rather than simply closing your post.  EWH

final Response.  Correct.  As stated above, I would not feel the need for HIV testing related to this event. 

As you know, we provide up to three responses for each thread.  This is my 3rd response.  I trust you will not feel the need to return to our Forum on this topic- repetitive questions on the same topic are discouraged and may be deleted without a response.