Pregnant.?

I had unprotected sex on October 3rd and 4th (he came inside me). Then a week later my period came. Ever since then I've been feeling sick constantly (hasn't been as bad these past few days), constant back pain, I'm constipated out of nowhere and i constantly have to pee. I took a test about 4-5 days ago and it came back negative. Is there a chance I could still be pregnant even without a positive result. I'm really worried :/