Hard Disk Monitoring

i woke up today to see my HDD disk health drop from 100% to 36% suddenly after doing a chkdsk /f /r /x on it (took about 2h for 400g of used space- around 600 open space) the health droped to 17% !!!
also i think i only had 64 bad sector at first and now in process of 3h it increased to 552 bad sector - it still has 100% performance and reading is completly fine (no files found missing - also tested with some games no crashes as now ) so do i need to consider swapping the disk now or is it fixable? (it says 23 day remaining life so idk)

I'm afraid yes, what you see is completely "normal" (even if not expected).

Generally any disk operation (file copy/move, defragmentation, antivirus scan, disk testing with Hard Disk Sentinel or any other tools) can reveal bad sectors. Depending on the drive and the usage, bad sectors may remain un-detected for very long time (even for years!) as explained at

www.hdsentinel.com -> Support -> Knowledge Base -> Hard Disk Cases -> Hard disk case: bad sectors
https://www.hdsentinel.com/hard_disk_case_bad_sectors.php

So generally it is good idea to perform disk testing, even on a new drive and/or if the drive is not perfect or when the Health % changes, as explained at
www.hdsentinel.com -> Support -> Frequently Asked Questions -> Hard disk health is low or recently changed or I just installed a new (used) hard disk. How can I perform a deep analysis?
https://www.hdsentinel.com/faq.php#tests

Just the type of test is important: chkdsk (even with the switches you wrote)
- does not properly check/test or "repair" the bad sectors,
- does not report any possible retries, performance degradations
- does not scan the complete disk drive (just a partition)
- does not report the actual location of the problems
and so. So I'd more recommend proper disk surface test methods (for example Disk menu -> Surface test -> Read test in Hard Disk Sentinel to reveal and Disk menu -> Surface test -> Disk Repair to fix / stabilize) instead.

I suggest to check this topic too for more information:
https://www.hdsentinel.com/forum/viewtopic.php?p=15974

The reported bad sectors are reallocated, so instead of them, a spare sector read/written.
So ideally what you see is completely expected: as the original bad sectors never read again, ideally (if you're lucky) all files should be readable and everything is "perfect".
This is absolutely not true - as increasing count of bad sectors mean increasing chance of data corruption / data loss: there is good chance that file(s) stored on a previous bad sectors (even seems readable) are damaged, corrupted.
There is also good chance that even more bad sectors will appear with time (similarly as now hundreds appear in a short time, after the previous smaller amount of bad sectors) and the spare area will become full.

Considering the age (1000+ power on days) and the low Health % value, low estimated remaining lifetime value, I'd backup immediately and consider replacement (at least if you have critical, important data).