Possible Twins

I took Clomid for two months. Found out we were pregnant a couple weeks ago. Have done Bloodwork three times to watch my hcg levels. First Bloodwork they checked my progesterone, which came back at 14.6 and they wanted it at least at 20. Put me on progesterone suppositories. At 6w4d I had an ultrasound done. They found two sacks! One big enough to see a baby at 6cm and a heart rate of 109. The other small and couldn't see anything. She thinks another implanted but did not continue to grow. I go back in 2 weeks to see if anything has changed with it. Part of me was hoping for twins, so now I'm concerned. Has anyone had such a thing happen before?