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Appearance of veins on breast, darker and larger nipples could suggest possibility of pregnancy. You should consider visiting your doctor/gynecologist again for a thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately). If the levels are very less, the tests can fail to detect pregnancy. Get the blood/serum HCG test and urine pregnancy test repeated.
Take care.