What is the best magnesium supplement to take?

I used to take oxide, but switched to magnesium citrate which I started Saturday night, and last night I felt very ill and didn't sleep much and tonight I am really sick with flu-symptoms - very tight throat, blocking nose, rhinitis, sneeze, dull headache. It's 5am here and I can't sleep at all and feel quite miserable!

I'm wondering if magnesium citrate is related at all to citrus from fruits? I have this kind of reaction to fruit, which is why I am wondering. It could be hayfever or a bug, but giving the timing I can't help wonder.