My dog is randomly barking at me, why?

For the past few days my dog has been barking at me for no apparent reason. She is a very affectionate and protective dog but has begun acting oddly and I'm not sure why. I took her outside to do her business tonight and she suddenly stopped sniffing, turned towards me and started to bark and back away from me - I hadn't done anything out of the ordinary and nothing was behind me. After a minute or so of this she stopped, turned around to do her thing, and carried on like it hadn't ever happened. This is the third or fourth time she has done this, and it only happens towards me and never my husband, nephews or other family. She just saw the vet and was given a good bill of health other than a flea allergy. What is going on here??