I have clear watery discharge coming out of my vagina, keep thinking it is my period starting because that's what it feels like but it's not?

See doctor: Any new increased or apparently abnormal vaginal discharge should be medically evaluated. At your age (17) STDs are high on the list of possibilities (gonorrhea, chlamydia, trichomonas, others) but of course only if you are sexually active. In any case, see your GP or perhaps your nearest NHS GUM clinic in the next day or two. Good luck!