My portfolio has been going down recently want to know why

Hello there I’m fifteen years old and last year I started investing with little knowledge and I invested in single company shares and I did make about 63 pounds in profit but then I learnt about ETFs so I decided at the beginning of the year to do 80/20 vti/vxus but for the past three week it was the greatest loss I’ve ever had and I lost about 40 pounds my portfolio is about 565.80. I want to know why this is happening and if there is any way to reduce the loss. Thank u