She fasts on Mondays and Thursdays with the intention that if she owes any outstanding Ramadan fasts then these are to make up for them, otherwise they are naafil fasts
If a woman is not sure whether a fast is obligatory or not in her case, then she intends to fast with an uncertain intention: if she owes any fasts, then this will be making up a fast, and if she does not owe any fasts, then her fast will be naafil. In that case her fast will be valid and will make up the missed fast, if she owed any fasts for days that she missed in Ramadan. However, what she should do is not make this the case in all of her fasts. Rather she should try to work out the number of days that she fears she must make up from the past, and try hard to absolve herself, thenmake a certain intention for her fast: either to make up for a missed fast or to observe a naafil fast. … She should not open the door to intrusive thoughts (waswasah) and doubts and uncertainty about her acts of worship. And Allah knows best.
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