All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The scholars differed in opinion about the minimum time-period of purity between two menses. The majority of the scholars are of the view that it is 15 days; whereas the Hanbali School of jurisprudence is of the view that it is 13 days.
The view we adopt here in Islamweb is the view of the Hanbali School of jurisprudence.
Therefore, if the bleeding is interrupted and then the woman sees bleeding again after 13 days or more, then this bleeding that she had again is menstruation.
If the bleeding returns in less than 13 days and the total period (of bleeding) when added to the period of purity which happens between the two bleedings exceeds fifteen days, then the days after the fifteenth day are considered Istihaadhah (bleeding outside menses).
Therefore, she should take into account her previous monthly period, and consider it as menses, and consider the duration (the extra days) that exceed this period as Istihaadhah (bleeding outside her monthly menses).
If she does not have a known and specific time duration of menses, then she has to act according to what she distinguishes from the bleeding; if she sees that the bleeding has the characteristic of known menstrual blood, with its color and its smell, and the pain that is accompanied with its discharge, so she should consider it as menstruation, and consider the rest (what exceeds this duration) as Istihaadhah.
If she neither has a fixed duration of menses nor can she distinguish the bleeding, then she should consider six or seven days as menstruation and whatever bleeding comes after this period of time is Istihaadhah.
In which case, she performs Ghusl (ritual bath) after the end of the days that she considered as menses, and then performs ablution for each prayer during the time of Istihaadah and prays with this ablution whatever she wishes of obligatory and supererogatory prayers until the time of this prayer expires.
Based on this, the bleeding that you see again after 11 or 12 days is not menses but it is Istihaadhah, and you should do what we have mentioned above.
For more benefit on Istihaadhah and irregular menses, please refer to Fataawa 345745, 380429, 362518, 145201, 326818, and 156775.
Allah knows best.