What Causes Repeated Menstrual Cycle?

Hi, I am 21 years old and I usually have a period every month they are anywhere from 28-35 days. This month the 28th day I had a little brown discharge when I wiped and that was it. Then on the 35th day I started bleeding brown but very lightly, and it is pretty much only when I pee or wipe. I am not on any birth control. Is something worng?