Has this happened to anyone? Went into ER for cramping and spotting was told they may be seeing two sacs?

So I went into ER last night for spotting and cramping. They did labs, everything was normal. My HCG levels were 5514. I know I ovulated on the 4th of this month, and got a positive hpt by the 13th so I believe I’m about 3 weeks along- going by the conception date.

When all labs were collected they sent me to ultrasound. She showed me One Gestational sack, which was measuring about 6 millimeters in diameter but nothing in the sac yet. I then saw another dark spot near it, but didn’t say anything because I figured the US tech would’ve mentioned in. When the Dr came in to give me my results, she told me I was still very early so she believed that’s why we weren’t able to see anything in the sacs just yet, and also that there was possibly another sac. It was measuring at 3.2 x 4.2 x 3.7 millimeters. No yolk identified in the second/smaller one. I wish I could’ve gotten a print out of it. I am doing blood work again on Friday to see how my levels are progressing and I’ll hopefully be meeting with my Dr to figure this all out.

So my main question is; if it was in fact twins... why would one sac be so much smaller? Has this happened to anyone? I’m lost and confused plus I’m a worry wart so not being able to be seen till Friday is going to really stress me out. I know im early but I had dreams of twins right at the beginning... which is why I’m blown away by all of this. Any advice, experiences, prayers, blood results at 3 weeks past conception always welcomed 💕