One day period?

I’m so confused. I got my “period” or what I thought was my period for one day. I took a pregnancy test and it came out negative. I’m having mild cramping as if I do have my period but every time I go to the bathroom there’s nothing. Last time I had sex was around 3 weeks ago so I would think I would be showing signs of pregnancy and not having implantation bleeding this late. I’m honestly scared, I don’t know what’s happening