Thin Yolk Sac

Jennifer
Has anyone ever been told during an ultrasound that one of the babies has a thin yolk sac?  Also, without a lot of fluid around it? 
I'm pregnant w/twins, one implanted about a week behind.  They both have a heart beat, but the heart beat she said she saw for Baby A (which implanted a week later) she said was kind of slow, and had a thin yolk sac with not much fluid around it.
I have so many questions.  I feel like the ultrasound tech didn't tell me much, maybe because she didn't want to upset me, but I can't stop thinking about it!  😫
Thanks in advance!