Why can't you lay down after taking flucloxacillin?

Excuse me, but I'm a bit confused about something. The paragraph I've read mentions that one should not lay down immediately after taking flucloxacillin. Could you please explain why this is the case? Is it because of potential side effects, such as dizziness or nausea, that could occur and make it unsafe to lie down? Or is there some other reason that I'm missing? I would appreciate it if you could clarify this matter for me.