Try resetting the clone tool.
With the tool active click on the tool icon in upper left corner then clck the arrow for drop down box.
There are a lot of options you could check, but sometimes the easiest way to make sure there are no esoteric combinations of options, modes, etc. active is to just reset the tool...
Right-click on the little Clone Stamp icon up in the options area and choose Reset Tool...
Edit: Curt beat me to it.