Is it necessary to worry about the validity of Umrah if a very small amount of blood was found after returning home, following a sensation of wetness during Sa'i, knowing that the age at that time was 16 years?

If a woman performing 'Umrah circumambulates (performs tawaf) before her menstruation, then performs sa'y, her 'Umrah is complete, even if she performs sa'y while menstruating. As for you, you performed sa'y while pure, so there is no need for doubt.