what reason could there be for transitory rashes?

As a child I had a severe case of eczema. There wasn't much they could do in the 1940's and 50's. The eczema resolved itself after the birth of my second child. About 7 years ago, I began having mysterious rashes pop up at various times. The first one looked like a burn and it covered my right breast and complete right side. Since then the rash has appeared on my upper chest, my face, my arms, trunk and legs. It doesn't look like eczema. It's bright red and slightly raised and it is contiguous with a distinct border. During this time I usually feel pretty tired too. It's not necessarily related to sunshine exposure. It just pops up every now and then. I've tracked contact allergens and my food intake and I can't seem to make any connections. Does anyone have any ideas???