Viral infections can do this, and are very difficult to diagnose. You might want to try taking a Vitamin C supplement (at least 500mg/day ~ work up slowly to a max of 1-3000mg/day) if you do not have kidney disease - Vitamin C fights viruses and reduces histamine. Also, you may want to try a low-histamine diet (no aged cheeses, wine, strawberries, etc. ~ look up "low histamine diet" online for specifics).
Also, has your doctor run a new set of allergy tests since this hives problem started? I ask because it is possible that you've developed a new allergy to something since your last round of allergy tests...
Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD (Lupus?), Osteoporosis, Anemia, T- & B-Cell Lymphopenia, malabsorption/malnutrition, Lyme Disease (Igenex Lab IgM WesternBlot positive/CDC negative), etc.
Meds: Pulmicort, Injectable Vitamin B12, Herbs, Nutritional Supplements, Homeopathy.