8 days late for my period . having slight cramps & upper back pain . tested negative 5 days ago . can sex stop period from coming ?

No: Sex does not cause a missed period unless you are pregnant. If you are pregnant, your HCG level may not yet be high enough to the threshold that the home pregnancy test was made to detect. You should retest in 2-3 days. Other reasons for a late period include stress, a change in your schedule, infection, or medicines.

Yes: If sex leads to pregnancy, then yes. Otherwise no, but a pregnancy can be helpful.