How can you lose if you buy when the price is low and sell when the price goes up?

I’m new to trading, and I don’t understand what happened. I had $3900 and purchased BTC at an average price of 90,000, selling it at 91,300. I also bought ETH at around 3,000 and sold it at 3,120 using a stop-loss method. Despite this, I am down by more than $300. Where did I make a mistake, and what caused me to lose my money? My portfolio was divided into 70% ETH and 30% BTC.