Pen tool - regarding combine shapes and subtract front shape

Hello Adobe community,

I have a question concerning the pen tool, specifically the "combine shapes" and "subtract front shape" modes.

My preference is combining shapes when making a selection and/or fill using the pen tool. I've been unintentionally hitting what I believe may be a keyboard shortcut, that switches "combine shapes" to "subtract front shape" resulting in a selection and/or fill of the inverse of the pen path/shape. 

Does anyone know the hotkey for this action? This has been a bit of an annoyance that i've tried to figure out on my own but have had no luck.

any help is greatly appreciated!

EDIT -

I have not yet found a solution but I have found the keyboard shortcuts in case anyone else has a similar issue. When in pen mode, it's + to combine shapes, and - to subtract front shape.

I use a cintique with the programmable EK Remote. I do not have - + assigned to any of the buttons so i'm still unsure as to what is causing this. I have the default alt, shift, ctrl and space keys assigned to the center four buttons which I use frequently. I'm wondering if it may have something to do with this.