I had the same issue, they told me there was excess fluid in her brain (they called it a chlorid plexus cyst), and said that a wile ago it was believed to be a symptom of trisomy 18, which is why they had to tell me about it, but that she was showing no other symptoms of it(her hands were opening and closing, her feet looked normal, her heart looked good,etc) and said it wasn't really something to worry about, but we would do a follow up ultrasound in 3 weeks. I of coarse worried like crazy until my follow up ultrasound and everything looked perfect, and her cysts were gone! Unless your doc seemed concerned,I really wouldn't stress over it too much, even though I completely understand how hard that can be!
I had the same issue with my son 4 years ago and it turned out to be nothing and he is healthy and a wonderful boy.. Try not to stress over it too much and try to think positive and wait for your follow up apt to see if any things changed
At 17 weeks my baby boy had a cyst ( but the type it was was a bunch of fluid) and it was in his brain. Typically they go away on their own. My baby's went away before the 2 week follow up with a specialist! I wouldn't worry and stress over it too much!!:)