Hello and welcome. Most abnormalities in the female body can interfere with the normal functioning of the menstrual cycle resulting in symptoms like menstrual irregularities, inter-menstrual bleed, etc. Although there does exist a possibility of this being due to the allergy you have been experiencing especially if you have taken any medications for it (which can easily cause irregularities), I would still recommend ruling out pregnancy using a home pregnancy test at least 2 weeks from the sexual contact or on or after the 2nd day of a missed period. Also make sure you use an early morning urine sample for the test for more accurate results. Best wishes.