What Causes Vaginal Bleeding With Huge Blood Clots?

Hi, may I answer your health queries right now ? I am 18 years old five ten and around 220. my cycle has been on for about 5 weeks and I have been passing huge blood clots.I went back and foward to the doctor; they told me that I tested postive for an STD. I have been taking my medicine correctly but I am still passing blood clots and having vaginal bleeding how long will this last, or is this normal to still be passing blood clots?( they found a small cyst on my ovaries also, could this be the problem)?