I had unprotected sex on my fertility days and it is now 12 dpo. My period is not due for the next 3 days but I have blood flow now. Could this be implantation?

How heavy is the blood flow? It sounds more like you're having an off period this month.

Wait until 2 weeks past your due date for your period, then test using first morning urine. If that test is negative, see what happens with your next period.

Sounds like your period is starting a few days early. Not unusual. If it is just a normal cycle, just a little early, that is all it is.

If it stops, and there is no period. Wait two weeks, and do a pregnancy test

If it's blood "flow", it is likely your period. Implantation spotting or bleeding, which not everyone will see but happens more than most people realize, is usually very light compared to period bleeding.

If you don't have a normal period, giving it some time to show just in case it is late, try a pregnancy test.