I recommend that you go to see your primary care doctor
about this issue, as there are at least a couple of common things that could be causing the rash, and the treatments are very different, meaning that a formal diagnosis would be helpful.
One possibility is that this is a yeast infection of the skin. These are relatively common on the breasts because the yeast grows best in humid environments. As such, the infection tends to start out under the breasts, but it can spread up over them as well. The other common condition would be some sort of allergic reaction, which could be from clothing, soap or other chemicals, or another exposure. Both conditions can be quite itching, so that doesn't necessarily help differentiate between them. However, your doctor will be able to tell, because the rash looks quite a bit different to the trained eye.
If this is a yeast infection, then a topical anti-fungal medication is probably what your doctor will prescribe. On the other hand, if it is an allergic reaction, then you may need a topical steroid cream to get the inflammation under control.
Please make an appointment to be seen when you can!