Phone battery suddenly dropping 30%

It's happened twice recently. I have a Samsung Galaxy S7. On Saturday, I was on my way back home from work and my phone was at 16%. I turned the screen off and literally as soon as it turned off it vibrated saying "battery critically low 1%" I was like how, it was just at 16%.

And then it happened again today not too long ago. On the bus back from uni, my phone was at 34%. I turned the screen off, immediately it buzzed saying it was at 2%. I really don't understand how in a matter of 1 to 2 seconds my phone can drop that much. I've restarted my phone too but it insisted it was that low. I'm not exaggerating, it drops in the space of time needed for me to press the power button to turn the screen off.

Has anyone else experienced this really sudden drop? I've googled it, but people are referring to their battery losing charge fast throughout the day (e.g. being 80% at 8am and then 30% at 10am). I'm talking about it going from a reasonable percent to almost dead in a few seconds.

Sounds like a battery problem. If you purchased the phone less than 6 months ago, then the shop needs to replace or repair at their own cost.

If still within the guarantee period, calls Samsung service.

I got it about a year ago so I'm not sure if it's within the period or not.