> But we put in a fixed amount of money
We did not put a "fixed amount of money". Money is created by both the Fed and by commercial banks. Whenever banks advance new loans, they create money out of thin air. Whenever they refuse a borrower to renew a loan, they destroy money.
When the Fed pumps money in the system (by QE), the banks act as a multiplier. But the multiplier is not fixed. The banks extend loans, businesses load up on debt, then they swim in cash, they do stock buybacks, stock go through the roof, people who own stocks feel they get richer (at least on paper), they consume more, businesses profit, then they want to expand, they go to banks, and the banks seeing how profitable the businesses are are happy to extend new loans, which is the same as to create new money.
It's a "virtuous cycle", depending on your definition of "virtuous".
A different way to look at it is that the economy overheats. The cycle of money creation has gone out of hand, and the Fed has to apply the brakes.
Yellen knows very well all these things. She was the head of the Fed, she is supremely smart. She's just lying through her teeth saying she "didn't fully understand". Of course a statement about her own state of mind is not a falsifiable statement, so she has the luxury to say whatever she wants. But she knows perfectly well how the economy works, it's absolutely impossible she didn't anticipate the results of pumping trillions and trillions of dollars in the economy.