Stay cool. If you have cholinergic urticariia you get tiny hives that grow & merge when skin temperature increases. This occurs with hot showers, hot weather, exercise & fever. Cooler showers, air conditioning, & tylenol (acetaminophen) for fevers will help. All antihistamines are useful but the best is thought to be periactin (cyproheptidine). In difficult cases i add Neurontin (gabapentin). Be prepared to be drowsy.
Zyrtec, (cetirizine) avoid heat. Avoid situations which will cause you to sweat and take Cetirizine (generic of zyrtec) 10 mg, 1 tab a day and increase to 2 tabs a day if needed. Also you may be able to desensitize yourself with a hot bath, producing a refractory period, during which hives won't occur, for 3 to 24 hours. If this is ineffective consult an allergist.