Jane:
What you're describing is called solar urticaria, or "hives from the sun." It's hard to treat/prevent, because when you're allergic to something (like cats, pollen), the best treatment is avoidance. Avoiding long waves of UV light (UVA) is tough, because sunblocks only take care of the short, burning UVB rays.
Suggestions:
Use a UVA sunscreen, like Shade UVA (there are many brands.)
Try other antihistamines: Claritin, Zyrtec, even OTC ones like chlortrimeton or Benadryl. See if any of them work better for you.
Wear light but tight;y-woven clothes over as much of your body as makes sense.
Consider visiting a major medical center which treats people like you by exposing them to incresing intensities of UV light, which can "harden" the skin against further reactions. I have no experince myself with this treatment, but if you're that unhappy, you might want to make the effort to look into it.
Good luck.
Dr. Rockoff