Missed periods. Know what is happening?

Doctor, i had an unprotected sex on 23 dec 2017 and i took ipill within 20 hours, after 5days i got sticky blood like spotting, then i bleed alot after 7days like normal periods but i don't whether it is or not because my actual periods is on every 12 to 13 dates and the blood is not as same colour as actual periods it pains like normal periods but not last long. But now i miss a periods on my actual date, it's already pass 8 days, tension and worrying develops on me i don't know what to do. is the pill not working or did i mess my period. I don't want to get pregnant now. When will i reach my periods again? Please let me know what is happening? And also i took pregnancy test on 6 January 2017it shown -ve, please doctor reply soon.