What's on your arm looks like ring worm to me. If that's what it is, you can use an over the counter anti fungal cream on it and it should help.

If you have eczema/are prone to eczema this could be discoid eczema or a ringworm.

If it only showed up while/ after you were in the shower than it is probably heat rash. My husband gets it when we shower together bc I like the shower much hotter than him! And we call it his ring worm (which its not). If it doesn't go away after you cool down then it is something else.