Is the fast considered invalid and does it require making up if there is a discharge with blood a little after the Maghrib call to prayer, knowing that the underwear was clean at the time of the call to prayer, and it was not checked with a piece of cotton?

Your fast is valid, and you are not required to make it up. When something is likely to have occurred at one of two times, it is attributed to the latter of them. Since these discharges could have occurred either before or after sunset, they are attributed to the latest possible time they could have occurred, which is after sunset.