What Causes Intermenstrual Bleeding?

OB-GYN Question I am a 25 year old female. Since March 2014, I have had several vaginal bleeding/brown discharge/spotting between menstrual cycles. The discharge is brown, opaque and looks similar to the final day of my period. I am a virgin and have never used tampons (so I can rule out STDs, STIs and pregnancy). I do not have any other symptoms other than the irregular bleeding/discharge. I had my first PAP in April 2014 and have had negative results for cervical cancer. The only lifestyle change I can think of is that my new job requires me to be on my feet for about 5-8 hours a day, I have to wear slacks paired with a belt (thus, my genitals are pressed/smashed against the pants fabric), and I have to change my lifestyle around the different shifts I work (from a 9am-5pm job to a 2pm-10pm job to a 4am-9am job). Based on the information, are the working conditions to blame? Do I have hormonal imbalance? Are periods ever evolving?