I had my last period on December 15th and started having sex on Decemb…

I had my last period on December 15th and started having sex on December 18th. I took a urine pregnancy test and it was positive. I visited the hospital a few days later and a blood test confirmed it was positive, but the ultrasound showed nothing except confirmation that I am pregnant. I tried to abstain from sex since then but had sex yesterday morning and all the symptoms of being pregnant disappeared. I can't feel anything again. Is this something to worry about or is it normal?