Am I sinful for praying in the mosque and fasting during the day, knowing that there were signs of menstruation?

These isolated drops are not considered menstruation by the majority of scholars, as the minimum duration for menstruation is a day and a night. Therefore, there is no sin in going to the mosque and praying. However, when the blood returned on the second day, reaching a total of a day and a night, it became clear that these drops were indeed menstrual blood. Consequently, the prayers performed before the return of the blood were not valid and must be made up.