I found myself in a similar situation recently. I had been taking the depo injection and had unprotected sex with my hubby in the 13th week too! I 'felt' pregnant, had tender boobs, felt sick all the time, really tired etc. I went to see my doctor because I was convinced I was pregnant but all the tests I did were negative. She told me it was very unlikely I was pregnant as the injection actually covers you for more like 14 weeks but they get you to come back for another injection after 12 to be safe. She said it was possible but not likely at all. As it turned out I came on to my period a few weeks later (about 2 weeks overdue).

How will you feel if you are pregnant?