Waalaikumussalam warahmatullahi wabarakatuh. Bismillahir rahmaanir rahim.
You will have to calculate 15 days of purity from the day you took a ghusl of the previous month (before you bled on the 12th). Then, note when the 15th day was and subtract it from the day you started bleeding (on the 12th) and see how many days are achieved between them. You will then have to make up for the Salaah you have missed during this interval. Your actual Haidh commenced on the 15th day that you have noted earlier.
If you have bled for more than 10 days (counting from the 15th day you marked), and you had thought it was your Haidh still and did not perform Salaah, then you will have to make up for those Salaah too as it will be regarded as Istihaadhah and no longer Haidh.
And Allah knows best.
Wassalam.
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