Why has my phone suddenly started overheating and draining fast?
Because something is running continuously that should not be — and the sudden onset is the useful clue, since hardware degrades gradually while software problems appear overnight.
The most common causes of sudden change:
An app stuck in a loop or failing to sync. A mail account with wrong credentials, a cloud service unable to complete an upload, or an app crashing and restarting repeatedly will consume the processor constantly. This is the single most common cause, and it usually starts after an update or a password change.
A recent app update, introducing a bug or new background activity.
Poor signal. A phone searching for a network, or holding a weak connection, transmits at maximum power — this is why a basement, a rural area or a train drains a battery dramatically. Airplane mode in a genuine no-signal area saves a great deal.
Background app refresh and location services, particularly an app newly granted "always" location permission.
Indexing or re-uploading after a restore, which is heavy and temporary — if it started right after setting up a device or restoring a backup, wait a day.
Charging while gaming or navigating, which generates heat from both directions.
A failing battery, where degradation raises internal resistance and produces heat — usually gradual, and visible in the battery health reading.
Case and environment, since thick cases and direct sun prevent the heat escaping.
How to find the culprit:
Check battery usage by app in settings, which names the offender directly in most cases and is the first place to look.
Note whether it is hot when idle, which points to background activity rather than use.
Restart, which clears stuck processes and resolves a surprising proportion.
Check for updates, both system and apps.
Remove recently installed apps if the timing matches.
Test in safe mode where the device supports it.
Why heat matters beyond discomfort: sustained heat is the main driver of permanent battery capacity loss, and devices throttle performance to protect themselves — so an overheating phone is also a slow one.