What Causes Missed Periods And Yellowish-greenish Vaginal Discharge?

I am 31 years old, 5'5" tall, and weight 125 pounds. My last period was on May 3rd. I have not had sexual relationships since April. I am having a yellowish-greenish vaginal discharge and no period. I took a pregnancy test last week and it came back negative (not pregnant). I had stress at work during the months of March and April but I am more relaxed now. Today I felt my usual menstrual symptoms (headache, nausea, tiredness) but not period yet. What do I have?