Period

My period came the 7th of July for just a day lightly. I want Monday the 10th of July and they tried to draw blood for hcg and the nurse couldn't get enough blood, so Tuesday they sent me to the hospital for blood test hcg and its was negative. Now Monday the 17th of July I have to go to my obgyn for ultrasound make sure I'm not pregnant cause my doctor said for me to have my period one day only it's considered implantation bleeding, can anyone tell me what's going on. I'm so scared.