helppp

I had intercourse on the last day off my (17th of may) period we used no contraception and he ejaculated in me twice. Now a week lateri am bleeding quite rich blood( meaning quite a bit of blood) ( it started on the 22nd of may).Could it be implatation bleeding? I am having low abdominal cramps, lower back ache, nausea. I done a pregnancy test today on the 23rd of may but i am still bleeding and the test came back negativ. Please help..