F1 to H1 B - Visa Interview. Question regarding MS


I did my MS in US in Civil Engineering and I am on H1B from Oct 1, 2012. I was on OPT for 2 years. My employer has sponsored for my H1B and I am continnuing to work for him on H1B too. My visa interview is scheduled for October 17 in Canada and I have a question here.

My major in MS is Civil and my work experience is in Computer Science. If the visa interviewer asks why you are working for a software company, when your major is Civil Engg. I dont have an answer. Would there be an issue for stamping H1B visa in my case? Please help me out.