What Causes Smelly Yellow Discharge During Period?

I am on my period and rather than it being red and bloody, as usual, it is yellow and is a snot-like consistency, in combination with an unpleasant scent. I also have a large, hard knot on my side, where my left ovary is. I have been unusually dehydrated the past week, drinking almost a gallon of water a day. I have not been sexually active for over 2 years now. There is no itching or burning. What is wrong with me?