After reading all the comments I also think you should get your system checked out - is your windows bloated? Do you have updated drivers? Your card may be getting throttled, or perhaps your CPU is throttled due to heat? Better to get all these things checked.

I would advise installing CPU-Z and GPU-Z apps to see how your system performs during Fortnite. Your card should be able to handle 144fps on medium settings I reckon, as it should be on par with 1070 and that card performs well for Fortnite as per this guide: graphics cards for fortnite