If school issue refunds on grants can they ask for some of the money back without warning that we may have to give it back?

I attend UMBC and I received a state Grant for school. When there is money left over from the grant after paying the tuition, the school refunds the money to the student. However now the school wants me to payback some of the money that they gave me 3 months ago because they may have given me too much. Am I obligated to pay back the money that was given to me without any documentation saying that in the case of a mistake I received too much I have to pay it back? I was given the money in September 2012 but they are just realizing this in January 2013, which is crazy, that means someone was not doing their job. As a student, I really know nothing about how much refund I am suppose to get so it’s not as if I could have seen the mistake. Can I sue the school to drop the charges?