Hi and thanks for the query, Its reasonable to suspect pregnancy in any missed period in a female of reproductive age, especially those with regular menstrual cycles. However, its not rare in some females, especially during certain cycles called anovulatory cycles, where ovulation fails to occur to have almost no bleeding or just spotting. The common pregnancy tests used in diagnosing pregnancy are only effective after at least two weeks of a missed period, it is therefore not unusual that your test could be negative. I do not actually think the antibiotics are implicated in you not seeing your periods. Exercising some patience and repeating the pregnancy test in two weeks using blood samples for the standard beta HCG test in a hospital setting or clinic would be the gold standard in clearing this doubt. Thanks and best regards, Luchuo, MD.