How can I know the reason of my F1 visa being revoked and B1 visa denied?

I was a student in the US on F1 visa in the period between early 2006 until late 2011. I graduated from school specifically in August 2011 and left the US the same month. My latest F1 visa was valid until 2014. But in December 2011 (4 months after leaving the US) I got a call from the US Embassy in my country (where my visa was initially issued) informing me that my visa was revoked, and I should apply for a new visa shall I need to revisit the US again. They did not give further details. I did not bother much about it, I thought it was just because I was done with my studies and I no longer needed the visa. Two months back (September 2012) I applied for B1/B2 visa in the US Embassy to revisit the US for a couple of weeks, but my visa was denied. In the interview I was asked whether I knew why my previous visa was revoked, and I answered "no". He also asked me whether I was arrested before or not, and I also answered "No". Then the Embassy officer denied my visa and told me that I'm welcome to reapply again, but probably I won’t be able to enter the US again!! Now I'm really concerned. I think that's unfair. At least I should know the reason for my visa revocation! I've never violated any immigration laws. I've never even had any traffic violation or any problems what so ever throughout the five years I spent there. And I'm sure there is some misunderstanding! Do you have any suggestions for me? Do you know the proper channel to contact the Department of State to inquire about the cause of my visa revocation? Should I hire a lawyer to help me? What can I do challenge this revocation and increase the chances of getting a visitor visa. I feel I've been treated with ambiguity and unfairness, the Embassy won’t give information, my previous school does not have information, and I'm really confused. I need your assistance.