May be pregnant, not sure? Help!

So I have always had fairly regular periods even while stressed! They have always been exactly 25 days apart. Well my boyfriend recently dumped me to be with his ex girlfriend who is pregnant (yikes) & while we were together we had unprotected sex using the withdrawal method. I am now 4 days late on my period, but all the home pregnancy tests I've taken have been negative. I don't know if it is just stress or what! Could I be pregnant and how effective is the "withdrawal method"? Any input helps! I'm about to lose my mind...