I have the exact same problem as the OP: After 5 (or so) days of taking a multivitamin, I come down with flu-like symptoms. Swollen glands in the neck & under the jawline, that "head full of cotton" feeling, headache, even congestion & sometimes a cough.
It's the damnedest thing. I've never heard of anyone having the same problem until now. My doctor has no idea what I'm talking about when I explain it to him.
So instead I take a C + B-complex vitamin with no problems.