What should I do and what is the expiation after performing 'Umrah with blood spots that continued for six days after the cessation of menstruation and ghusl, knowing that I was in a state of ihram and had taken pills to delay my menstrual cycle in Ramadan?

The returning blood is considered menses because it occurred within the period when menstruation is possible, and the purity between the two bleedings is a valid state of purity during which acts of worship are permissible. Since you performed tawaf after purifying yourself and taking a ghusl, and you did not see blood until after your return, your tawaf is judged to be valid, and your Umrah is valid, Allah willing, and nothing is incumbent upon you.