What Causes Itchiness In Vagina And Burning Sensation While Urinating?

im a 16 year old girl im 5'9" and 180ibs i have just started using birth controle and ive recently been sexually active. My partner has cheated on me multiple times with the same girl. I recently have been experiencing itcheyness in vagina, burning when i pee, and dark red period blood. My period is not very heavy like usual but its so light i only have to wear a panty liner. I dont know whats wrong with me please help!