I can't say anything about Warsaw bloc countries but I can surely say about was going in early 90's after Soviet Union dissolution. Because - a) I was there, b) I researched the topic quite a lot.

1. Actually the fertility rate was falling quietly but steadily alongside the rise of urbanization.

2. In many countries such as Ukraine the production of consumer goods has fallen in the 90's yet there were no deficit (empty shops) due to even greater loss of purchasing power. Basically in Soviet times they had more cash but goods production was lacking and after that situation reversed. Shock doctrine - liberalization of markets - pushed most of the people into poverty.

From what I've read about Poland - shock doctrine also contributed negatively to socioeconomic factors (at least initially). There was a decrease in GDP in 1990 of 7 percent and the same decrease -7% in 1991. So drop in fertility wouldn't be that sudden after all.