Hi there,
Usually, if you see a noticeable difference in your vaginal discharge, you should seek advice from a gynecologist. The soreness you experienced could be due to lack of lubrication and dryness during or after sex. The yellowish/brownish discharge might be an indication of an infection; if it continues you should definitely have it checked out by your doctor. Sometimes antibiotics (and other meds.) can cause vaginal yeast infections, because they can often change the level of bacteria within the vagina. If you think that the antibiotics might be the culprit here, speak with your doctor about your concern with the antibiotics that you're currently taking...
Take Care