How reliable are condoms.

We use condoms EVERY time, and as far as we are aware none have broke. My cycle is anywhere from 30-35 days usually. Last was 29 days. 
Today would be day 39. I am usually pretty good at knowing when my period is coming. I have endometriosis so I pay close attention to my body and usually get horrid cramps but not this time. I've had very minor almost not there cramps for about 4 days. No spotting. Nothing. I know its a little early to be worried but im starting a new job and moving so I wanna know as early as possible. Im not stressed by any of this. Im super excited. I graduate tomorrow so I dont believe its stress related. Sorry its long but I'd like opinions and feel more information helps. Lol