What would cause an itchy, painful rash over collar bone?

Plant juice, maybe. There are several things that could cause such a rash, especially if the rash is only on one side, not both sides. Shingles is a possibility. I'd expect clusters of small blisters to develop in that case. A contact rash from a plant or chemical, either alone or combined with sun exposure could cause this. Even bacterial infections, like impetigo or fungal infections, like ringworm, are possible.

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