What Are The Chances Of Pregnancy Inspite Of Being On Depo Shots?

I have been on the depo shot since January, starting on a smaller does then and I ve cut it close to when I can get another one, although it hasn t ran out yet. I was put on a progrsterone for 10 days to get rid of symptoms I was having but they re still there and after looking it over, they seem like pregnancy symptoms. I don t know when I would have conceived and I don t know if it s possible for birth control to run out early. Any ideas?