Coming off birth control can be a tricky situation, some times it can really throw your periods off. Mine were never regular after I got off birth control, although they were always regular while I was on it. So while it's possible that you are pregnant (some women get pregnant the moth after they get off BC) it's also possible that you're not, and that the BC is working it's way out of your system causing some irregularity in your periods along the way.
I say wait a little while longer, a couple of days... and take another test. If it comes out negative again, you're probably not pregnant. Sore nipples could also be a symptom of your period since your hormones are telling your body that it's time for your period.
When you come off the birth control pill, it can take your body some time to regulate itself again, and you might not have a normal period for a couple of months. However any time you have unprotected sex you are at risk for pregnancy. So I would wait a week, take another test. If it is still negative then you can go to the doctors for a blood test. I didn't get a positive until I was 5 weeks. Good luck!
Sorry, spelling correction (bloody iPhone). Exactly 28 days later, like clockwork ....