yep. panau. I had it before. I went to a skin specialist. It also known as a fungal infection but you know, it's not like those icky fungus or anything like that. lol. It's completely normal. But definitely see a specialist to get medication and of course practice proper procedures like applying sun block as well as clean your skin often. If you can. carry around tissues to wipe off sweat. Some skin are just more prone to this.
I would also advice you to lightly exfoliate the skin with let's say, clinique's clarifying lotion. This product contains salycilic acid which is a mild exfoliant so that it helps to even out your skin tone in the long run. I still have a little bit of my panau marks but it has decreased when I started exfoliating.