What Causes Light Menstrual Bleeding Despite A Negative UPT?

I am 32 years old and my period and period symptoms have always been the same every time. I always have severe PMS symptoms with horrible cramping, breast soreness, and the first day of my period I always pass a big blood clots with heavy bleeding with my period lasting anywhere from 3-5 days. I have always been irregular with my cycle ranging from 31 to 40 days. On July 5th I had a normal period. On July 25th and 26th I had unprotected sex using the pull out method. On August 11th through the 15th I experienced dull cramps occasionally and that was about it. My period was very light but dark brown the entire time with some light red mixed into it. My period was so light I didn t even need to wear a tampon. When I put a tampon in it would come out dark brown with light red scattered through it and looked like clear cervical mucus in it as well. If I stuck my finger in my vagina it had brown and red but nothing was really coming out. I m confused if I even had a period. I took a pregnancy test on August 11 and 14 which came back negative. What are possible causes for a dramatic change in my period color, amount, and symptoms? Yesterday my breasts were perky than usual and today both seemed like an off and on burning or hot sensation. Other than that I have had no signs of pregnancy symptoms or menstrual symptoms. Is pregnancy possible? If so how likely is it possible?