I've had this problem before, just like you. It seems that the selection is so small, that you are still dragging one of the nubs by accident. Try and look if your cursor is like this when you're dragging it. It should be like this: (but without the cross and black arrow). Is there a smilie for this cursor? I couldn't find it.
If the selection is so small that you cannot make the cursor change, try moving the selection with the arrow keys, since your image size is 16x16, it should be quick enough.
Hope this helps.
*EDIT*
I just went ahead and took a screenshot of the cursor, so you know what I'm talking about. Your cursor should look like this: . Remember, sometimes the selection is so small, that it's impossible to make it change. In that case, use the arrow keys to move the selection.