there could be multiple reasons that this is happening.
1. your sim could have just returned from a freezing lot
2. you could have took a cold shower/swam in the pool too long
3. you could have drank 20 glasses of water (which is very unlikely but still possible)
4. it could just be a glitch
ways to fix:
1. buy a thermostat
2. drink a lot of coffee
3. get in a hot tub or take a steamy shower
4. save immediately and wait it out. If your sim is still freezing or dies in two sim hours, restart back at the save point and do some other options.
