Why does rice go bad next day?

Leftover rice is one of the most common causes of food poisoning. Once cooked it needs to be immediately cooled down if you aren't going to eat it straight away, and it needs to be kept cold until reheated. Reheating should be a quick process that doesn't allow it to sit for a long time at a lukewarm temperature.

Rice can be a very dangerous food.

It's always been recommended to me very strongly by my in-laws (Japanese, the family all together eats close to 1kg of rice a day) that rice, if not going to be eaten immediately, must either be left in the rice cooker on the 'keep warm' setting, or stored immediately in either the fridge (if you plan to eat it the next day or so) or the freezer (for keeping it up to a month after originally cooking it). And by stored I mean wrapped tightly in cling wrap or placed in a tupperware container or ziplock like bag. If storing in the fridge or freezer, it must also be prepared and stored straight out of the rice cooker while still warm. Do not leave it to cool on the bench before throwing it into the fridge/freezer, because that's when the rice will start to go bad.

Now, I don't have any scientific research to back up these claims, but I have seen the above advice repeated on various cooking related shows and by many a housewife here in Japan, so I would assume that they know what they're talking about to some extent.

Still dont think that should cause it to go bad so soon as Ive always

That's the problem with your line of thought, right there.

Your rice has always been unsafe. For safe rice, you have to cool it down to under 4 Celsius within 4 hours of being cooked.

Unsafe food will sometimes spoil, sometimes won't. It will sometimes make you ill, most of the time do nothing. And the two conditions - "spoiled" and "will make you ill" are only weakly correlated, you can get sick when unsafe food looks and smells perfectly OK.

Predicting what bacteria will take over unsafe food, or retroactively explaining it, is as difficult as predicting the weather. So a literal answer to your question (why did it happen now when it hasn't happened before) will take mountains of data and weeks of developing an appropriate model. Also, there is no way to give you advice under which circumstances to expect it - as long as you don't follow food safety rules, it can happen any time.

I noticed that the rice I cook lasts longer without spoiling than when my mom cooks it. I realized that it’s because I rinsed the rice more times than my mom did prior to cooking.