What Causes Yellow/green Discharge?

I have yellow/green discharge, I am in the week of my peroid and usually it is very heavy it has been almost not even there but spotting. I went to the doc and the preg test was negative. I'm also having realy bad cramps. I had sex with my fiancee' on the 1st and the 8th my peroid usually starts around the 15th which it was also a couple days early.