Hi. So, when we are on the pill, we don't 'really' get periods. The pill manipulates our hormones so we don't ovulate at all (required for a real period) and during the 'off week' of pills, that cessation of hormones in our system causes spotting. As hormones are added, taken away, we can have break through bleeding or spotting.
Another possibility, do you have any itching? When our discharge is pink or brown, brown is old blood, pink is new light blood mixed with our discharge. So, you could have some irritation that is mixing with your usual discharge. Does that make sense?