hello, You did unprotected sex around fertile days and there is quite chance of being pregnant. Brown discharge before period due date is mostly suggestive of implantation bleeding. Here, you need to undergo one blood test for beta hcg & TVS scan to rule out product of conception after one week or following missed period. Early test is less significant. You can consult with gynecologist after getting test reports for next line management. Wear tight breast support to get relief from breast discomfort. Practice safe sex in future. Good luck.