Yeah, this incident is on Rurika, not Sera. Rurika declined medicine, said it's probably period cramps, and sent Sera off with a smile. Then Rurika spent the whole day feeling like hell and refusing to tell her wife that anything, to point that she ended up in the hospital without Sera knowing anything was even out of the ordinary.

I really think Ruriko do everything right I mean why would she tell Sera to go to hospital when Sera said she's so excited to go having fun if you checked the page 13 Sera didn't goes straight to home instated she start to think about stickers which I think is the time when Ruriko collapsing even when Sera back she thinks Ruriko will welcome her at home like every time.
The thing is Sera doesn't consider Ruriko like a wife and she goes having fun alone without Ruriko she even travels alone.
Check Chapter 1 when Ruriko got so happy when Sera bought bread for 2 persons I think Ruriko trying her best to not be
a bother on Sera so she's don't break there life even through it's one sided love .