...but when I tried to add to it, the freeform pen tool icon got a little minus sign on the bottom right...
You only get that minus sign on the cursor when your cursor is over an anchor point.
...and insisted on reshaping everything else I drew...
Correct. Removing anchor points will indeed reshape the path.
... What is that little minus sign and how do I get my freeform pen back to drawing additional freeform shapes on that path?...
Its an automatic 'feature' whereby in order to speed up editing the path you can quickly delete anchor points, the minus sign when your cursor is over an anchor, or add anchor points, when your cursor is over a segment.
It really does depend on what you are going to use the path for as to whether or not the Path Operation needs to be set. If you intend to just stroke the path then the operation has little if any effect, but if you intend to create a shape then it makes a big difference.
As you haven't said what it is you intend to do with the path I suggest you do as IamSam mentioned and make sure the Path Operation is set to 'Combine Shapes'. (Ignore the fact you are creating a path...menus do not differentiate between the two.) You can of course always change each sub-path Operation later should you need to.
When adding to an existing path DO NOT click on an anchor point or segment of any existing path, start from a blank space and overlap your new path over the existing.
If you do use different operations then these will still be reflected in the Paths panel, regardless of how you proceed.
Regards.
MrToM.