Hi,
Welcome to the forum. I am really glad to help you out. The following causes of the vaginal bleeding need to be looked into.
This could be due to fluctuation in hormone levels, hypothyroidism, genital infection & cervical polyps/erosion. Injury to one of the blood vessels causing the bleeding is less likely as the surgery has been done 3 weeks ago. Please do consult a gynaecologist for the same as it is not possible to diagnose online without a physical examination and investigations.
I sincerely hope this helps. Take care.