I recently had rash due to Tegretol and ended up in ER. They prescribed two different antihistamines: Pepsid and Benadryl; the idea is that the two block different "histamine channels", which is why you need both just in case. I think you can get both over the counter; Pepsid needs to be taken before food - probably 30 minutes before, but follow the instruction. I would get those immediately as they cannot hurt and might help. I have been on Lithium for years; larger doses gave me acne, but on lower doses I am OK. I never had rash, but acne is a type of a skin reaction, so...

Very sorry to hear about it.