If one feels that some of their previous prayers may not have been accepted for reasons such as: praying just before the time expired, or not doing ghusl when one may needed to have done it (due to doubtful mani/mazi issues)... can the person make intention of praying qada for an amount of days to make up for the prayers that they have confusions/doubts about?
And in case, there was no qada left for the person, will the prayers be considered nafl?
In the Name of Allah, the Most Gracious, the Most Merciful.
It is highly discouraged to entertain mere doubt and conjecture, as mere doubts and conjectures are generally whisperings of Shaitan; and if a person entertains and acts upon such doubts and conjectures, the whisperings of Shaitan will increase and become stronger. If you ever get some doubt, it is advised that you consult a scholar.
If however you have genuine reasons to believe that certain salats that you offered may have been invalid, then you can go ahead and do qada of those prayers.
And Allah knows best.
Mufti Faizal Riza
Darul Ifta Australia