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Your symptoms of breast pain, headaches, gassy feeling and bloating along with delayed periods suggest possibility of pregnancy. Urine pregnancy tests can show false negative results. Look out for other symptoms suggestive of early pregnancy like increased sleep, hunger and tiredness, mood changes, headaches, fatigue, nausea, constipation, nipple sensitivity, blue veins on breasts, back pain, lower abdominal cramps, increased frequency of urine, pelvic discomfort and heaviness, food cravings, dizziness, increase in vaginal discharge. Go ahead with examination and blood HCG level tests done by your gynecologist to confirm/rule out pregnancy.
Take care.