pregnancy this soon after delivery while nursing?

I delivered my second son on March 6, 2012 and I breast feed exclusively so my period has not yet resumed. However I had experienced weird brownish/pinkish discharged about a week ago followed by cramping. I thought it was my period sense I had acne break outs, lower back pain, and gas all typical pms symptoms. Yet no actually period. Starting today I have noticed a white sticky discharge and increased back pain. Since I am breast feeding I do not know if its normal to be this sore or if its something else. I took a pregnancy test which was neg. but not sure if its too soon to test. I am really confused why my body is acting weird and wonder if I am crazy for testing for pregnancy as it is. Is it possible for me to be pregnant this soon after delivery while nursing? or is this just my hormones balancing out? Everything look good at my six week post delivery so not sure why I am experiencing pms/pregnancy symptoms with no period and neg. pregnancy test. Any Advice?