Hellow Doctor, Last 27th Februery Me And My Partner Had

hellow doctor, last 27th februery me and my partner had a unprotected sex, but i had ejaculated ouside of her vagina, but the next morning few drops blood came out of her vagina, is this implantation blood? is she pregnant? but today 29 februery early morning she took a test on prega news the results are negetive. Did the preganews workd properly? becoause she tested after 3day after the sex.