The bleeding after sex was likely spotting from some stretching or erosion of your hymen. It's normal, it happens, and your body doesn't need to "get used to sex." There is simply often tissue there around the opening that gets little tears in it.
Chances are you haven't gotten your period because you're so scared and stressed out. Do see if you can relax some. if you used a condom for ALL genital contact, the whole time, and it did not slip off or tear, you can rest assured it was 100% effective as birth control for you.
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson