Hi there
I am a manager and have to receive/deal with these self cert/sick notes and fit notes on a regular basis.
You may self certify for 7 consecutive days. It doesn't matter what hours/days you work over these days, but day one will be considered as the first day you are off sick from work.
After 7 consecutive days, you will need to provide a Dr's note advising if you cannot attend work. This used to be called a Sick note but the new style is call a fit note - the idea being you tell your Dr about your job and if you are able to do SOME of your normal duties, the Dr writes on the note that you can do XYZ etc and you return to work to only do these duties until the Dr certifies you are well enough to do more or all of your normal duties.
If you are not able to carry out ANY of your duties or of you have something contagious etc and it is recommended that you don't attend work at all, the Dr will write this on the note and you need to send it into your boss.
If I have read your info correctly, and this is day 2 off for you, then day one would have been yesterday - Monday - and Yesterday will have been the first day you have taken sick. Therefore If you do not return to work next Monday, you will need to provide a Dr's note covering you from next Monday onwards, as you can only self certify for 7 consecutive days - i.e. Monday through Sunday inclusive.
Hope that makes sense? Sorry it's long winded but hope it helps, and that you feel better soon.
Jet