I've been experiencing sore and itchy breasts for a week or so and now have a red pin prick rash on one of my breasts can anybody help please?

Hard to answer: I feel you need to see your primary care doctor to see the rash and then he will determine what is the right course of action.

Hydrocortisone cream: I would first try a Hydrocortisone cream for a week followed by a review of all oral and topical medications or creams used as well as avoidance of sun exposure. If this does not improve in 7-10 days , i would see a dermatologist.