What Cause Bloody Vaginal Discharge Between Periods?

i have bloody discharge in between periods every month. i went to the gyn last month after it happened and got checked from just about everything, but apparently its not an std...she said i had a lot of "white cells" but they said if the gyn doesnt call you, then the results were all negative...no call. btw...i am 23 yo, 5'5, 135 lbs. i had clamydia 5+ years ago, and i've had 10-15 uti's in the last 8 or so years. ive always had a lot of discharge, but everytime i get tested, they said nothing's really wrong.