Hi averi147,
welcome to the pple (user-to-user) Community!
Re: "My phone sent me a notification and somewhat later I have less screen time".
Wondering what the notification said, before the screen time changed.
and
which iPhone model you use, and, which iOS version is installed.
ScreenTime use may be completely controlled by yourself, unless there are parental control in place.
See: how to: Use Screen Time on your iPhone or iPad - Apple Support
If parental controls are in place, you may need to ask a parent for help, as only the person with the password can make changes:
If Family Sharing is used: Set up parental controls with Family Sharing on iPhone - Apple Support
or
See how to: Use parental controls on your child's iPhone and iPad - Apple Support
Hope this helps you find the satisfactory settings you would like to use for ScreenTime.
________________________
Else: Trouble-shooting if needed:
What to do if you (or a parent) forgot the ScreenTime passcode:
https://support.apple.com/en-us/HT211021
All the best :-)