What Causes Menstrual Bleeding With Long Duration?

Hi!I assume that i have some problem with my health lately. 3 month ago i have moved to Chicago from Kiev ( Ukraine). We dont have such a big climate difference, so i dont think its a kind of acclimatization. I have 26-29 days period, duration 5-6 days. I never had any problems with it. Last 3 month i ve had flow on 8-10 day of period ( just a little bit and only for one day, looks like period starts). Can it be due to vitamins? i am taking vitamins for skin and nails. Or you think i have problems and i should visit a doctor? Thank you in advance.