Last normal period was 3rd of September. Lasted 5-6 days, had unprotected sex on 14-17th September. This morning had like a very small amount of stringy brown blood, haven't had since, could this be implantation?

Yes it very well may be. It may be just spotting that happens once in a while, a sign of a infection and other things, but implantation is one option that it could be