I need advice

Br

Today I went in for a ultrasound. I’m only 5 weeks. And they said they saw the gestational sac but no yolk sac. My doctor said my hcg levels were a bit low. Isn’t it too early to see the yolk sac? He mentioned that I could possibly have a miscarriage but he also said that I could carry a baby to full term. This is my second child. Has anyone else had this happen? Or am I just freaking out over nothing