Cold/cough symptoms with blister 3 year old

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Hi, my 3 year old DS has had a cough and ear infection for just under a week, we went to the GP this morning as the ear infection was getting worse and he was prescribed antibiotics. He has developed a few spots like little pimples, he also has a blister on his back, these spots developed before taking the antibiotics.

Anyone any idea what the blister could be? Just I've never heard of blisters forming with a cold.

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Has he had chicken pox? That comes with cold like symptoms

Yes he had chicken pox earlier in the year along with his older brother.