What Causes Greenish Vaginal Discharge?

Hi, Ive been going to the doctor about trying to conceive since Oct 25, 2010. He put me on clomiphene starting at 50 mg. I took 150 mg this month. I also had him check me for infections because i had a great amount of cervical mucus that was greenish and brown. He gave me something for yeast and it cleared up. Around the 14th day of my last cycle it came back but only last for 2 days. Can you explain to me what's going on in there?