nepalikanchhu bro,

Yes, I was also employed through consultancy. Even I've a degree in Comp. Engineering and they didn't ask why I changed my field. I don't think it's a problem unless you either have engineering/computer science/IT background. As mr.august suggested, ofcourse you've to be well-prepared for the interview to tackle any kind of questions. The consulting company should have clean history. In my case, they asked me where I work and if I like that place. They also asked me to describe the job that I'm doing. Like I said, if you prepare all the required docs as mentioned in US embassy website, I don't think you should have any problem unless you yourself have clean background history too.