What Causes Missed Periods While UPT Is Negative?

Doctor.plz help me m scared...I missed my periods..its 8th day today..in between the missed periods days I had sex 3 or 4 time..last time I had sex yesterday..and today s morning I checked the result was negative..what is the matter..kya missed period days k bich me sex krne se pregnant ho skte haij kya,but maine aaj subah hi check kiya to it came out negative and line was dark..plzzzzz help me plzz