TMI/Confused

I had implantation bleeding (light pink spotting) 3/20 on the day I should have from my cycle. Period due 3/21 (25 day cycles) The next day I took several pregnancy test. And 2 days later a couple more 1 really faint, but a couple clearly positive. Today 3/24 I started bleeding. Really mid cramps. Not heavy, but appeared darker then normal, flow a lil less then a normal period.. I went to the ER... They drew my blood, comes back and says “your blood test was neg for pregnancy, you just started your period a few days late” and sent me home.. like, wtf like how could this be? 🤔🤔 I thought HCG was only present during pregnancy..