An infection including viruses or bacteria
The depo shot begins to work 7 days after your period. In those seven days you are protected from pregnancy. Because basic math, 16 plus 7 is 23. Your time is up by the 23rd of having unprotected sex. Unfortunately yes, you are pregnant. Lol I'm an accident too. It's dreadful :)
OD. All of the above, I believe.