What Causes Vaginal Bleeding After Sex While Having Copper-T?

my wife had copper T in the month of March. After that, for 3 months after the period she had blood bleeding for 4-5 days. We consulted a doctor. He checked the copper T position and said it is in place. Now she has little/traces of bleeding after we have sex. is this normal due to copper T or some thing need to be checked?