My filing status in 2013 changed to single, my status in 2012 was head of household. When I file 2012 CT taxes, can I use head of household?

Your actual circumstances in 2012 determine how you file your 2012 return. Changes that occur in 2013 have no bearing on your 2012 tax return.

I believe that Connecticut is one of the states that requires you claim the same filing status that is filed on the federal return, so, yes, claim Head of Household for the 2012 CT return.