Losing my hair?!

My hair has always been decently thick. It doesn't grow fast at all though. I went to get my hair cut and when he was combing my hair he said "hun you have some bald spots" I flipped out and he showed me I have a bald spot by my ear, on top of my head and a couple in the back. They are about the size of pennies, but it scares me. I'm only 20, what causes hair loss?! I'm not overly stressed, I feel like my stress level is average. But I'm super concerned right now. Any advice would be amazing 😩