still no period, help

hi i still havent had my period, last one was August 4 n i was due for my next one on August 31 n nothing yet.. i did a urine preg test n it was negative. i went to the doctors n my gyn did a vaginal exam n said she could not tell if i was pregnant thru the exam, she sent me to get a blood test done due to some abnormal bleeding n spotting n also for pain n of course the results were negative.. today is sept 15 n still no period, i havent had any spotting but i did have severe left lower abdomen pain, kind of felt like my fallopian tube, weird.. the pain has gone away but I did return to my gyn who did another vaginal exam n said that he did not feel any lumps or mass but that my uterus was tender n sent me to get a pelvic ultrasound done.. any clues to what might be wrong with me?