Suggest Treatment For Menstrual Irregularity

I'm 31, and started having my period one month after delivering my daughter, and was regular untill 2 months ago, but haven't had it since. I've done three pregnancy tests all of wich were neg. I'm currently nursing full time and have been from the start. I'm experiencing lower back (wich radiates into my hips and down the tops of my legs) and pelvic pain, nausea, fever of approx. 101, fatigue and general feeling of malaise.