Help!

I logged into my Itunes account with my username and password and it seemed to be working fine, but when I tried to exchange an Itunes gift card (and then a prepaid card) Itunes asks me repeatedly to log into my account even though I'm already logged in (because my email address appears on the upper left corner of my Istore page). Then, occassionally, a page appears that says "we cannot fulfill your request, make sure the date and hour in your computer are correct or enable cookies from Itunes" , and I've checked both and they're ok, so I don't know what's happening. Help?