2 year old with rash

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My 2 year old son has had a rash for about 3 weeks now it's all over his stomach/chest and all of his back, just tonight some spots have appeared on one of his cheeks. He's been totally well no fevers no being irritable been completely himself eating/drinking as usual but now I'm getting worried what this is. I'm going to doctors tomorrow with him but would like some suggestions on what this might be? Thank you in advance xx