Im on birth control and i had unprotected sex on cycle days 16 and 26 (placebo week) and took plan b both days and now im 3 days late and took a test and came out negative, should i be concerned?

Pregnancy unlikely: Home pregnancy tests are very accurate and not affected by foods, herbs, common illnesses, sexually transmitted diseases, or medications. A correctly done urine test gives a reliable result 3 weeks after sex (3 weeks after when conception might have happened). Emergency contraceptive pills can alter the menstrual cycle bleeding for the current cycle and sometimes the next cycle.