pjv.....When you opened your account at Schwab, you signed an agreement (it's in the fine print) that any stock you have in "street name" (you do not hold the stock certificate) they can lend out to a short....if there is stock available.
If you are short a stock you are not the owner of that stock, it is only temporarily lent to you until you cover that position. So they can not lend anything you are short, as you are not the holder of that stock, only the borrower.
If you are able to short in your account that means you have a margin account. Even if there is enough money in your account to cover the entire position in your account, because it is a margin account your position is able to be shorted . The only way a stock can not be shorted is if you have the stock certificate mailed to you. And that is only if you are long the stock.