What causes cold urticaria and solar urticaria in the same patient? Could it be an hhv6 infection/encephalitis she had at 18 months?

Urticaria. It's not known why these two conditions occur. They may be inherited or caused by a virus. In the case of solar urticaria, there is also activation of some antigen in the skin by ultraviolet light which then causes an ige reaction. Ige is an antibody responsible for immediate hypersensitivity reactions of which urticaria is one. The mechanisms and causes of these are not entirely worked out yet.