Well, it turned out to be sad news for us. Fetus had grown the whole week and was twice as long as it was on the previous scan, but then its little heart had stopped, maybe just few hours before the scan and there were no cardiac activity shown in placenta, neither.
Doc said that his opinion is that the additional sac looked like another gestational sac which had stopped development earlier, maybe up to two weeks ago. He said that there seemed to be smaller, younger fetus inside which had passed away and that's why the sac had started to go distorted.
So I had a missed miscarriage at 8+2 weeks yesterday. Sometimes it goes that way, sad but true. I'm already a mother of two beautiful, healthy children so I'm not completely down with this. I hope I'll be more lucky next time.
Thanks for the comments and pics, everyone, and all the best for you and your families!