Hello and welcome to the forum. I am sorry that you are having this fear and these issues! Please know that checking with a doctor is always best. When is the last time you had your annual well check visit? That would be an excellent time to discuss breast tenderness. As you know, that is common as our hormones fluctuate. And so are skin issues. Estrogen, for example, is very intertwined in our skin. As it fluctuates throughout your cycle (and your life time), you will see effects. Estrogen keeps our skin hydrated and helps with its elasticity and oil production. Progesterone is another hormone as it fluctuates that can impact skin. In coordination with estrogen and androgens, fluctuations of progesterone can lead to acne, dermatitis, and pigmentation. I would imagine that a doctor may check your hormone levels to make sure there is no imbalance as well.
Rashes under the breast can be caused by heat very often. That crevice is hot and warm. Here are some other causes of rash under your breast. https://www.healthline.com/health/skin-disorders/rash-under-breast. Cancer is listed also in this article and I know that is your main concern. I do suggest you go to your doctor to rule that out but your description does not sound like this is going to be the outcome. The symptoms of inflammatory breast cancer are as follows:
-pink or red skin discoloration
-pitted skin often described as looking like an orange peel
-an inverted nipple that points inward rather than outward
Remember that inflammatory breast cancer is rare.https://www.cancer.gov/types/breast/ibc-fact-sheet Again, your symptoms do not seem to match but please speak to your doctor. Let us know if that is possible.