What Causes Abdominal Pain,pain During Sex And Irregular Period?

I am 24 years old - I have a number of symptoms including dull abdominal pain, pain during and after sex, irregular periods with brown discharge in between cycles and dull lower back pain. I was treated for having PID a year ago. I have always had "spotting" in between periods and considered this to be normal but because this is now accompanied by a range of other symptoms I am starting to worry. What could this be? I would appreciate your help, thanks.