FPS significantly decreased without any obviuos reason

I face really unorthodox issue with my fps. Usually I have around 180 fps at the beginning of the game and above 90 during all the game stages (1v1). But yesterday my FPS was 100 at the beggining and around 60 at mid-late game. Nothing changed in hardware of software configuration. After 40 minutes break I tried to run 3v3 and my FPS was around 180. Next game 1v1 - 100 FPS once again. I also had some slight lags even though my internet connection seemed ok. Guy whispered me after the game and said that I cause lags.

I run out of ideas what might be the cause of this behavior. Maybe someone might give me a clue. Would be grateful.
This is what I tried to do to fix the issue (nothing helped):

  1. Checked the temperature of CPU, GPU and motherboard via software and via my finger.
  2. Downloaded new GPU driver.
  3. Checked my CPU and RAM usage during game.
  4. Rebooted my PC in case there was some memory leaks.
  5. Changed game mode from fullscreen to fullscreen(windowed).
  6. Was running ping command during game to track package loss and latency spikes.
  7. Scanned my PC for viruses.

My PC specs:

CPU - Ryzen 5 5600x

GPU - Nvidia GTX 1070ti


2 monitors - 165 and 144 Hz