What Causes Delayed Periods While HPT Is Negative?

Hi doc.. Me n my gf had sex on 18th jan..her periods was got on d date 3rd dec..aftr dat it comes early on 21st dec..but problm is dat we had unprotected sex on 18 jan..and its 8 feb yet her periods not comes...and last 3-4 days i hv checked 2 times pregancy check kit device which indicates negative sign...now...my medical friend sujest me epforte..can u plsss help me out this doctor??