I had eczema for 10+ years (tanning, actually, finally got rid of the last of it a few years ago), and what you have sounds more like a heat rash, especially if it's popping up when you're hot or sweaty. However, since you asked about it on your thighs, I did have a patch of eczema on one of my inner thighs for a while. If skin is there, eczema can be there is a general rule of thumb.
But if you don't have a rash or irritated bumps or blisters, it really doesn't sound like eczema. If it is a heat rash, just wash the area with a cool, damp cloth, and let it air dry (don't put clothes on over that area until it's COMPLETELY dry). Don't let moisture get trapped on your skin. You should see improvement pretty quickly.