My mother has had severe hives come and go for months. No allergies after testing. What could this be. And could she be allergic to her cigarette smok?

CHRONIC URTICARIA. The cause of hives is unknown in up to 50% of patients! Some patients have 'chronic urticaria' which has to be managed with histamine blockers such as zyrtec (cetirizine) and zantac (h1 & h2 blockers). She should stop smoking for overall health reasons.
See MD. Chronic hives are a very difficult medical condition. A patient with hives for more than 6 weeks is categorized as "chronic". Such a patient needs a thorough evaluation to look for unusual causes of hives -- from the very benign to the very serious. As mentioned it is not unusual, however, for no specific cause to be found.