What Causes Delayed Menstruation During Treatment For Fever?

on 5th feb my due date of priod and got cough, cold & fever on 31 jan i took antibiotics and pain reliver for 5 days. during medication 3 days continue i had blood spotting and stoped whn i stoped my medicine now its 15 days i didt have my period. during this time i took 3 prego tests which is negitive. whts the reason for delay??