Extremely thick white discharge a week before my period was due, it wasn't clear but sticky. three days later i had brown spotting.

Infection? STD?: Any new, abnormal, or unexplained vaginal discharge should be professionally examined. Thick and white suggests infection, which also can cause spotting between periods. At age 21, STDs like gonorrhea, chlamydia, or trichomonas are among the most common causes, so get it checked out ASAP. Your local NHS GUM clinic would be an excellent source for expert, lifestyle-sensitive care. Good luck!