NT ultrasound was normal but found possible fibroid?

Roxy
I just had my NT ultrasound this morning at 12 weeks 4 days and everything with baby is healthy and normal! Whew. 
What was unexpected was a mass/growth that the sonographer found that she originally thought was a fibroid but ended up doing a transactional ultrasound to make sure and she said that it doesn't look like  anything she's seen because it is large and fluid filled (I guess fibroids are usually more dense, no fluid) 
She asked me about past periods and pain but nothing unusual on my end. Now I'm freaking out that it's something that is cause for concern. Good thing is I don't think it's baby related, bad thing is I have no idea what it is and what it could be:benign cyst? Malignant tumor? 
I've been so concerned that baby is ok it didn't even dawn on me that something could be wrong with me. But if there is makes me worry baby isn't fully safe either. 
Anyone else have any experience like this? I know it's not common but any advice would help.