What are some other explanations for spotting other than implantation or the start of period?

What are some other explanations for spotting other than implantation bleeding? I am 9 days DPO according to my calculations. Yesterday (8days DPO) I had a browinsh-pink bit when I wiped so I automatically put in a tampon and went about my work day. On the tampon later was just a little browinsh discharge. Wore pantiliner today and last night with nothing. I had backpain Saturday and Sunday if that means anything. Although I have unprotected I couldn't possibly be pregnant fiance had vasectomy 3-4 years ago and it was successful. Would love to be preggers but impossible. Wondering about another reason for this irregular spotting.