What Causes Irregular Period And Recurring Vaginal Infection?

hello my name is daniela am 18 i missed my period every other month and am not pregnant sometime i feel pain in my belly and back and am seeing this white thing coming out of my vagina and it makes me hitches and i went to the doctor and they said the its a infection but this is the 5th time now i'm seeing it and am tired of it every time i go to the doctor the gave me a type of bill to insert in my vagina but it don't works my period are now missing and i don't know what to do i need some advice please. i only have sex with one person in my life