What Are The Chances Of Pregnant After Using Protection?

I am sexually active but ALWAYS use condoms. My last period was during the second week of June (I belive around the 11th but i'm not sure of the exact date). I have not got my period yet but with school ending, working, and being stressed out I'm not sure if that is what is causing it to be late. I have been sexually active this month but have used condoms every single time. I also have all the signs I normally have of PMS.. cramps, break outs, being emotional, etc. Could stress really be causing this? (Like I said I have used a condom faithfully and it never broke).