Can implantation bleeding begin day after sex? day after having sex, had light bleeding(3 days) with one or two small clots. does that mean pregnant?

Doubt it: It takes a couple of days for sperm to find/join with the egg in the Fallopian tube.It takes a couple more days for that mass of cells to move into the womb & attach. If you had light bleeding the day after sex it likely has nothing to do with conception & implantation. You might eventually be pregnant, but I wouldn't view these findings as a reason to suspect it.