So upon your return to work, you had an investigation for absence I'm presuming, it is normally carried out immediately after the return to work. Your manager, at this point due to process, decided to move the procedure onto a disciplinary hearing.
You will be invited to a meeting by letter and you must have 24 hours notice of this, the meeting must be held on a day you normally attend work and you may take along a representative if you wish. If you aren't in the union, another colleague can attend. You will be asked about the absence, and any other absences you may have had. The manager that conducts this, which will be a different manager, will then decide whether to issue you advice and counselling for sickness, as this is your first step on the process for sickness, or not based on the information provided. (There will also be a notetaker present).
If this does happen, you can appeal it by writing to the deputy store manager within seven days outlining the reasons why you think the decision was unfair.
However, for one period of absence you shouldn't have been invited to a disciplinary hearing. How long have you worked for the company?