I've Come Down With A Severe Case Of Hives, Under

I've come down with a severe case of hives, under my arms and in my groin...Last night my doctor suggested 4x5mg of Prednisone, followed by the same this morning. The hives went completely away. Yet around 4:00 pm today they started coming back. The only thing we can figure that changed in my diet were some sugar free cookies with Splenda. I am NOT diabetic, but have a family history so I watch my sugar intake. I had a lot of the cookies yesterday and again this afternoon. Any thoughts on Splenda being an allergen, since I injected more of than usual?