You could have taken a due after 4 weeks for a conclusive result. You can take an antibody after 12 weeks for a conclusive result, so the retest choice is yours. An early test can't provide answers, otherwise the guidelines would say to test at 8 weeks.
If you are comfortable with unlikely then that is your choice.
A one time exposure of unprotected vaginal sex is low risk, however, it IS a risk. You need a conclusive result. Very true that your 8 week test result is unlikely to change but you could take a 4th generation Duo test and know for sure. That would be conclusive. Otherwise, you should test one more time at 90 days to confirm your negative. It will likely not change but since you did actually have a risk, you should make sure.
I know that results are conclusive only at 3 months marks.
But I want to know from experts like you have seen 8 week negative later turned positive?
Is 8 weeks a good indication?