Hello dear I understand your concern Antihistamine does not cause delay in period. If you did sex during fertile phase of period then pregnancy should be rule out. Brown spotting could be due to implantation bleeding and it will occur around the period date. I would suggest to go for urine pregnancy test and or blood HCG test to confirm pregnancy. In absence of pregnancy, delayed period could be due to hormonal imbalance, ovarian dysfunction, excessive stress, change in life style and dietary habit I would suggest to consult gynecologist and undergo examination, reproductive hormone analysis, USG scan to confirm cause Progesterone pill can be used to induce period in absence of pregnancy. Avoid stress, take healthy diet, drink plenty of water and do regular exercise Hope this may help you Best regards Dr. Sagar