What other symptoms are you having? Does tha rash itch?
Use calamine lotion for the rash.
Usually an infective process like mastitis; or trauma to the site or breast abscess or cystic disease of the breast are painful lesions.
You could have pain in your breast associated with hormonal changes linked to your pregnancy .
You could try some OTC pain relief medications, warm compresses and see if it helps with your symptoms. Wear lose clothes, and try some breast massage.
Just follow up with your physician for a detailed clinical evaluation and that should put your mind at ease.
I think you should go ahead with an ultrasound if your physician advises, so that a confirmed diagnosed can be got.
Let us know about how you are doing and if you have any other doubts.