Congratulations on your new PC, Spooky and also, if this is your first time here, welcome to the forums as well!
Perhaps view it as your gaming experience for Overwatch has improved tremendously as compared to your performance going up exceptionally because of the new PC. Maybe that might help to put things into a better perspective?
The next step to to get them shiny new RGB enabled peripherals!
With regards to SR, Kaawumba made an informative thread here and it may be helpful to you:
My own experience with placement SR has been a little wonky, so you are not alone.
In season 10, i did placements for fun and my record was 2W-7L-1 Draw and received 1564 SR, while for season 11 placements, my record was 5W-5L and i received 1469 SR. For season 10, i played just 1 more game on top of the placements and the final SR was 1546.