Oprah Winfrey Doubles Weight Watchers Investment In One Day

10/19/2015 3:31 PM PDT

1019_oprah_weight_watchers_TMZOprah Winfrey made $45 million Monday ... off of one stock.

Oprah announced this morning she bought 10% of the outstanding shares of Weight Watchers -- a $43.2 million investment.

The stock soared from $6.79 Monday AM to $13.92 at the close of trading, grossing her a cool $45 mil.

The announcement Oprah was investing in the company made investors go crazy ... the volume of shares traded was 70 times more than normal. 

Amazing.