I remember seeing something exactly like this on reddit about two years ago. I'll try and find the thread at some point today if I can.
However, it it just a monitor issue, or does the same effect happen in screenshots and/or screen recordings? Also, something that comes up quite a bit when I googled it is making sure your refresh rate is correct, but I don't imagine that'd've changed without you noticing. Try fiddling around with the response time on your monitor, if you have such an option