Getting pregnant after depo

Ok so I was on the depo shot for about a year. I got off of it in July and the day after I missed my shot I got (what I thought was) a period . It lasted several days and what not...but I haven't have another one since. My boyfriend and I are very sexually active and we don't use pertection, for the last 2 weeks or so my breast have been very sore and I've been having some cramping ...almost like I'm getting a period but I'm not sure if I'm pregnant. And how long does it usually take for depo to get out your system ?