I had unprotected sex on oct 4th, which is 6-8 days before my period starts. on oct 18, i had a light brown spot.is this implantation bleeding?

Do pregnancy test: This could be implantation bleeding, it would be the right timing. If you already had your period on Oct 10th-12th than the bleeding on Oct 18th probably isn't implantation bleeding but if you missed your period, you could be pregnant! Do a pregnancy test and see what it shows. If you want to be pregnant, good luck! Your test should be positive by now.