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If you have been sexually active after your last delivery, then chances of pregnancy cannot be ruled out. Since you are not breast feeding the child, the protection of lactation amenorrhea (suppression of ovulation due to breastfeeding) will not be effective.
You should consider visiting your doctor/gynecologist for a thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately) and ultrasound scan done to confirm/rule out pregnancy and also confirm gestational age of fetus if pregnant.
Take care.