Auto fill happens automatically when I don't want it to

I hope someone has had & solved this problem.
Sometime when I double-click in a cell it auto fills the the rest of my spreadsheet down to the last used row that has data.
This very frustrating as it overwrites everything in it's path.
I have a lot of VBA macros in this spreadsheet, but nothing that auto fills columns or rows for that matter.

Thanks Dave,
I don't think that's it since I'm usually clicking text or numbers and I haven't noticed the plus sign when it happens.
I'll try to pay more attention.
I do use a larger than normal mouse pointer. Maybe it's too large?
Thanks for your response.