Negative battery cable hot.

After sitting for about 2 months, I tried starting my ‘71 Duster 340. It was hard to start but after some starting fuild it started. After warming up the engine I turned it off and tried to disconnect the negative battery cable. I normally do this because I don’t drive the Duster. The negative battery cable burned my hand. It was extremely hot to the touch. Prior to this the car ran fine and the battery cable never got hot. Any help to fix this problem would be greatly appreciated. I normally start the car once a month. Could it be a bad ground? Bad battery cable? Bad battery?