I got my first and ONLY depo provera shot on 9/28/11 and I was supposed to go back on 12/12/11 to get it again but I never did.. While on it, I missed my period in October but got it in November and December and twice in January.. I had unprotected sex a few days ago, so does that mean I could be pregnant?

Anytime you have sex, especially unprotected sex, pregnancy is going to be a possibility. Odds are that you aren't, however, but you will have to wait and see. You may not be able to rely on a missed period as a symptom since the depo provera can cause your cycles to be off for quite awhile. After a few weeks, you could try a pregnancy test and see what results you get.

Having unprotected sex with anybody will end u up pregnant I mean nothing no pill can keep up away from that especially when u haven't gotten the second shot.

Since my periods were already irregular, how do I know when to do a pregnancy test?

You will have to wait that three or four weeks from the date of sex, and start checking at that point if you are not sure when period due