Period started 9/25 ended 9/27 now i've been having sore breast. my period isn't due until 10/24. what can cause this? sexually active no protection

Do a quick HPT: Home pregnancy tests are easy to do, fast, and highly accurate. A healthy woman in her reproductive year can become pregnant any time during her cycle, higher chance during/around ovulation and less so during active menstrual flow. So, check a hpt to find out. 2-3 days of short/light period common in early pregnancy. It is good chance that you are pregnant. Consult doc. Good luck.