Can I get a doctors note after the fact?

I've been off sick this week pretty sure with this flu virus thats going around - I was planning to go back tomorrow but I'm still kind of feeling run down (bad cough, runny nose etc). I breach 7 days today so I'll need a sick note if I don't go back tomorrow, but am I still okay to get one tomorrow and apply it retrospectively? Or did I need to do it in advance?

thanks!