The only way to know for sure is to wait until you miss a period, if you even do. I know thats not what you wanted to hear, but its the truth.
Birth control can make your periods highly irregular. That could be the reason for the lighter than normal period. That could also contribute to the sore breasts. We get extra hormones in our body when we are getting ready to start our period. That, mixed with the hormones in the birth control, certainly could make your breasts sore.
There is a chance of pregnancy with sex, protected or not. No birth control is 100% effective. If you miss your period, take an HPT using your first morning urine about a week after the missed period, or go to your doctor and request a quantative blood test. Try and relax and remain calm. Stress can cause your period to be delayed
