New pc lagging

Sep 8, 2018
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I built a new pc 2 months ago
Amd ryzen3
Geforce gtx 1050ti 4gb
8ram
And suddenly my pc is lagging a lot in games i don't know what to do this problem it seems very weird but i can't find a solution
 
It's start to lag in games that i was playing for example i play fortnite in full graphics and now it's lagging too much and it looks very weird and this happened to many games that i used to play without lagging