What could this be?

It doesn’t usually happen in summer but in fall or winter if my cleavage gets sweaty I will get this rash. It itches and burns so bad. This is it on a good day, I’ve had it all over my cleavage and under my breasts before. This has been going on for about 4 years. Whenever I put jock itch cream on the area it seems to help it. I just would like to know what’s causing it.