I hope others will chime in because I'm bad at remembering details when I'm engrossed in a show. But I believe she momentarily falters, slightly collapses perhaps. Then, in the subsequent scene(s) her left hand is shaking and she supports it with her other hand. I was surprised by it because I've seen the play before, but it didn't really change anything. It was a physical manifestation of the shock Lena was feeling. I don't think it had to be in the script. Unless someone helped hold her up when it happened (which could be, I'm not sure), that's all the notice that was given.