I have rashes develop just below both breast, feeling very sore...

Rash: Rash under the breast is most commonly from fungal infection. Try Monistat Cream and antifungal powder. If it does not get better, seek the help of your physician.

Rash: It could be a contact dermatitis, due to a reaction from your bra or other clothing, or even the detergent your clothes were washed in. It also could be a simple heat rash, due to moisture under the breasts and excessive sweating. You can try a little over the counter hydrocortione - apply a thin layer twice daily. See your doctor if no improvement in 5 days.