Should I fast this day after having performed ghusl following menstruation and intended to fast before Fajr, then found a light discharge and do not know when it appeared?

If what the questioner saw was blood, it is considered menses if it occurred during the possible time [for menstruation], and she must make up that day. However, if it was yellowish or brownish discharge, it is not considered menses unless she saw it during her habitual period, and her fasting is valid in that case. If it was natural discharge, it has no effect on the validity of the fast.