It's been three weeks since i had sex, my period is 8 days late but i've had 2 negative pregnancy test, could i be pregnant or is it something else?

Could be: pregnancy, but more likely something else. Home pregnancy tests are fairly reliable and if you have had 2 negative tests, then it is unlikely that you are pregnant. If you period is only a week late, then it could be that the tests were done too early to detect the pregnancy, so repeating one more test in 2-3 days to be sure, is reasonable. Many things can cause a woman to miss a period.