Im 20 yo male and have brown spots on chest and neck. They don't hurt, neither are sunburns. What can they be? I developed um after losing 50lbs. But even though I'm healthy now. I still have marks. Please help

I do lift weights and eat meat and healthy, what can the problem be?

I have brown spots on my chest and neck, they don't hurt, just look weird, I have about 20 spots, they are random shaped. Please help