Headache after Whey Protein Isolate?

I have some good quality whey protein isolate (BiPro, no additives)that I bought a while ago and recently started taking a little (yes its still within expiration). I only take a half scoop (10g of protein) mixed with some milk with my lunch. I feel pretty good for about 2 hrs, like my circulation feels good or something and then 2-3 hrs after ingesting it I start feeling a headache come on and my sinuses and nasal area swell up. Any ideas why this might be happening?