Bleeding between periods.

Thursday 5/29 was exactly one week from the end of my peroid. The night I started having horrible back pain and pain behind my knees, and later on abdominal cramps that were waking me out of my sleep. The next morning I started bleeding, bleeding not spotting, and was having to us super plus tampons! (Usually only use super) it's slowly has come to a stop and I'm just having very light bleeding now. This happened in March, but much light, and my doctor told me to just "monitor" it and that it's probably from stress. I'm starting to think not. Anyone had this happen before? And what was the outcome? I've had regular 24 day ish cycles for the past 9 years so I'm not really sure what to think of this.