Alarm bells may ring when you first read this reply but dont panic, what you've described actually sounds like and STI - Chlamydia to be precise. With chlamydia, women usually experiance a thick discharge throughout the day, and during sex it does become very painful, the brown colour in the discharge is blood, and often with chlamydia you will bleed after intercorse and bleed irregularly throughout the day without any intercorse. Remember, being on the pill does not protect you from any sexually transmitted infections, and if you do decide to have unprotected sex with a partner, you must consider having regular tests, some of which (when being tested for chlamydia) you can actually do at home, others take place at walk in centers and your hospital, which test you for everything (pain free). Make an appointment with your GP as soon as possible and notify them on your concerns, and they'll point you in the rite direction. Do this as soon as possible, although im not 100% it is chlamydia, it does sound like thats the case. Take care and i hope i helped you out xo