Why did my put option lose money when the stock went down?
For the same reasons a call can lose while the stock rises. A put's price pays for three things: where the stock is against the strike, the time left (which shrinks every day), and the size of move the market expects (implied volatility, which often falls once news such as earnings is out). If the stock fell less than time and the lower volatility took away, the put lost money. And the price you can sell at (the bid) sits below the middle price.
Try it: a put on a $50 stock, strike $50, 30 days left, priced at 50% implied volatility (made-up numbers). Move the stock down, let days pass, let the volatility fall.
Simulated example, priced by the StrikePath engine (Black–Scholes, interest 4.3% a year). For learning; not advice.
A put is insurance, and insurance has a premium
A put gains when the stock falls, by roughly its delta (about half a dollar per dollar for a put at the stock's price). But part of its price is time value, what you pay for what might still happen, and it shrinks every day. Another part is the expected swing: when the market stops expecting a big move, puts get cheaper at every strike, as insurance gets cheaper once a storm has passed.
The fall you actually needed
Held to expiry, a put gains only once the stock is below the strike minus what you paid: the put in the example above needs the stock below about $47 at expiry. Before expiry, it depends on the time and volatility left, which is what the demonstration shows.
What this shows, and what it doesn't
It shows how a put can lose value while the stock falls; it can't tell you what happened to your own. To look at a real loss, compare the same contract at two moments: the stock's price, the days left, the implied volatility, and the bid and ask. The split depends on the order the changes are applied, because they interact; together they always add up to the total. One volatility for every strike here; real prices usually make puts below the stock dearer (skew).