Empty Gestational Sac at ultrasound

So I had some very light pink spotting and so I went in to have a vaginal ultrasound. According to my LMP, I should be 6w3d. I had bloodwork done a week and a half ago and my numbers were good. However, when I went in for the ultrasound they just saw a gestational sac. My doctor said she just thinks it’s an early pregnancy and my dates are off. She’s putting me on progesterone and I go back in on Monday to see if there’s growth. She said based on my numbers, I was where I was supposed to be according to the ultrasound. I just don’t have a very good feeling. I tracked my ovulation last month and my period was regular. So idk how my dates could be off. I will say with my son my dates were off. Has this happened to anyone else?