First U/S today..

Went and had my first ultrasound today, baby is measuring at 6w5d and has a strong heart beat but I was told I have a very abnormally shaped gestational sac. It's supposed to be pretty round and mine is nowhere near that. 
I was told that could either mean I had twins and one didn't make it or that something else is wrong. She told me to take it really easy and to watch out for any cramping or bleeding, I'm just so terrified something bad is going to happen now :/. Anyone have experience with this?