This is the problem with scans these days, they see too much and then have to tell us. I am sorry for what you're going through, I haven't had the exact experience, but we did have a problem with our scan and they "had" to tell us even though the same thing would never have been detected in the years before. I was worried sick like you, they told me on the friday and said book in for an u/s next week. I ended up being on the phone to different Dr's all weekend, and finally one rang me and said "it's really a very minor problem, they shouldn't have scared you, but they have to tell you".
I hope the outcome is fine for you, I just wanted you to know that they pick up so much more on the 20 week scans these days, and often it is not a major problem. Can you ring and insist on speaking to someone about this? Don't let them put you off, talk to someone who knows about these things. I hope you can relax soon.
Take care xooxox