What Causes Brown Vaginal Discharge After Starting Birth Control Pills?

My son is 10 years after him i started birth control pills...then i completed my family n I have been using birth control pills from 1 year after my 3rd baby ....I started my birth control pills on 5 day of my periods..but after that dark brown discharge started....its now my 8 day but still having same problem but the color of discharge is now lil light....some times vanishes but then starts again...this never happened before...... Is it normal?... N can I do unprotected sex now?