I Have Been Having Cramping And Lower Back Pain For

I have been having cramping and lower back pain for about 2 weeks now. My last "period" came 4 days early and only last for 3 days when it is normally 5-6 days long. After what I am assuming was my period, I had brown spotting for a week. Now I have thick, milky white discharge. I still have the cramping and the lower back pain and I'm pretty bloated. I went to the doctor and he said he didn't know, and ordered me an ultrasound that's not for another 3 weeks. Any idea what it could be?