Fluid in ultrasound?

I'm trying to stay calm but I just got a call from the nurse at my doctors office and they told me they found fluid in the ultrasound (13 weeks). They said the heartbeat was strong and normal but that I needed an urgent referral to an OB. That's all they told me and now, of course, I'm in panic mode. I really don't want to freak out but I have no idea what this could mean or how severe a situation I'm in. Has anyone been through this? Can you share your experience (the good, the bad and the ugly)?