For the last couple of days i've had thick white discharge. i've never had it before my period before. what does that mean, if anything?

Depends.: Most importantly, if you have had unprotected sex and are not in a long-term mutually monogamous relationship & you have both been checked and found free of disease, then the possibility of an infection exists, see doc. If that is not so, and you have no itching, fever, or abdominal pain, wait a few days and see if it goes away. if it does not, see your doc. Stop any new supplements or otc meds

You may need to: check for a vaginal infection. If there are no other symptoms, such as vaginal itching, odor, etc. then you may be starting irregular spotting/ periods with your hormone cycle at your age. It could be very early on, or minor spotting may be a fluke. But if it continues, you may need to see a GYN or family physician to help you regulate your cycle to be more predictable.