Should I suspect hair loss if my scalp becomes more visible when I tie my hair up?

Lately, when I tie my hair up, the crown of my head feels thinner than before. It stands out even more under the lighting, so it bothers me for no reason.

There are days when I lose a lot of hair, but it doesn't happen every day, so it is difficult to tell if it is just a matter of my condition or if my hair is getting thinner.

I’ve heard that when it comes to hair loss, you need to look not only at the amount falling out but also at hair thickness and scalp condition. What changes should I check for in the early stages? I would also like to know the criteria for seeking medical treatment rather than trying to solve the problem just by changing my shampoo.