no, you cannot extend the previous period of sickness as you returned to work. the new period of sickness starts on the day you first had off, you self-certificate in the same way as usual an then need a doctor to sign you off after that. as you returned (even if it was only for 5 minutes!) to work, this will all be treated as two periods off work. the lesson is to take as much time as you need off and not risk doing this - it does happen, but in the future, take your sweet merry time about it.