That's how my eczema presents.

I get random rashes sometimes, that i think are contact allergic reactions. I also get rashes from the sun if I'm in it for too long. I don't know if this would be the same as what you are experiencing, but my granddaughter has rashes that get worse when exposed to the sun, water, and if she exerts herself, especially on her face. Her dermatologist told us that it's not eczema but dry skin. She uses Eucerin rough and bumpy lotion (whole body) and Amlactin cream (at sight of rash) at night. As long as she uses them daily, it stays under control and nearly undetectable. If she doesn't, the rash never goes away.