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 rulings of menses depend on the discharge of the known blood of menses regardless of its quantity. Allah Says (what means): {And they ask you about menstruation. Say, "It is harm, so keep away from wives during menstruation.} [Quran 2: 222]
In the above verse, Allah made the rulings of menses based on the blood of menses.
The Prophet said to Faatimah bint Abu Hubaysh: “If it is the blood of menstruation then leave the prayer. After passing that period you should wash the blood from you [i.e. perform Ghusl] and perform the Prayers.” [Muslim]
The Prophet considers the beginning and ending of the period for the implementation of the rulings of the Sharee’ah. He did not consider the quantity of the blood, whether light or heavy.
Therefore, as long as you observe the blood, you should abstain from Prayers and fasting until you become pure.
For more benefit on the difference between monthly menses and bleeding outside the monthly menses, please refer to Fataawa 129256, 199375, 192925, 183343, 154565 and 145201.
Allah knows best.