From your symptoms, you seem to have developed you seem to have developed yeast infection.
From increased sweating, and wearing tight clothes yeast gets accumulated producing infection in that area. And yes from increased itching the skin in that area has become very sensitive. I suggest you first stop applying anti-itch cream. Breast contains sebaceous glands; therefore there will be production of sebum and will be collected continuously on breast. That's the reason you are prone to infection.
You need to maintain proper hygiene, especially in breast area. Try taking bath twice a day. Try not wearing too tight clothes. Try wearing loose clothes, where there is good air circulation. Do not wear your bra in the night. Use anti-fungal cream twice a day.