I self certified for a week, needing a sick note from saturday. (sunday also being one of my working days) My doctor issued a note for two weeks but was dated the Monday.
if the note says "2 weeks" rather than a specific date, then the note should run for 14 days from the date it was signed by the Dr. Obviously, as Factor30 says, don't go back unless you're well enough!
Yir Doctor hasn't said you're fit for work, if he'd thought that, he would have put a date on the med cert ("You should refrain from work until <date>".... and then you're fit FROM that date.)
The two weeks run fae the date the Med Cert was signed so he should expect to see you either to issue another med cert or confirm you are fit from the day after the current one runs out..
If you go back to work before the end of the med cert, you will not be covered if anything happens at work because you were not fit for work as per the med cert.