Why Is Doxycycline Prescribed For Vaginal Spotting?

My Doctor prescribed me doxycycline hyc 100 mg and metronidazole 500mg for 7 days twice day. I am wondering what this could be used to treat? I went to the gynocologist because I had pain in my abdomen and spotting. He said he didn't think it was a STD, but hw is testing me for them anyway.