Everyone is sensitive to different things...I have never had a reaction to Splenda, myself.
Just to double check, you might want to think back on something that you might have eaten or done otherwise in those same time frames-used a certain lotion/beauty product, or another food? Just to be safe.
My daughter had a bad reaction from lemons, I from codeine (I itch like mad for HOURS) and my husband has a reaction from something in Reese Cups.
He isn't allergic to chocolate, or peanut butter...but there must be some preservative, or small ingredient that he has a reaction to. His cheeks get flushed and hot every time he eats one.