What Are The Chances Of Pregnancy Having A Very Light Bleeding?

Hi I'm 20years old and i'm three days late on my period but I've been having pains like period pains for a couple of weeks and I have been feeling very tired and I have also had a headache for two days now.. me&my bf had sex last night and after when I went to the toilet I bleed but it was like a light period and I haven't had anything since. I took pregnancy test today and it was negative. Could I still be pregnant?