What may be understood from your question is that you did not get the usual menstrual bleeding , even though this was the time for it, but you got a single spot of blood . A spot of blood is not regarded as menses, because menses is blood that flows from the uterus, so it cannot be a mere spot.
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) was asked: If a woman gets a few spots of blood during the day in Ramadan , and that continues throughout the month of Ramadan and she is fasting, is her fast valid?
“Yes, her fast is valid. As for these spots , they are of no significance, because they come from the veins. It was narrated from ‘Ali ibn Abi Talib (may Allah be pleased with him) that he said: These spots of blood that resemble a nosebleed are not menses. This is how it was narrated from him (may Allah be pleased with him).” (60 Sualan fi’l-Hayd, p. 6)
And Allah knows best.