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If you have monitored your ovulation time and had been sexually active during your most fertile days and ovulation time, chances of pregnancy are possible. The symptoms (increased tiredness, headaches, bigger/fuller feeling breasts, abdominal cramps, brown discharge) that you describe could suggest pregnancy possibility. Wait and watch for your next periods. Look out for development of any other new symptoms if pregnancy is suspected. If periods do not start within 10 days of expected period dates, or if you get abnormal periods, or if you suspect chances of pregnancy, consider visiting your doctor/gynecologist at the earliest for thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately).
Take care.