Thanks guys. I'm glad it's not just me. I was wondering if my IP address had been blacklisted somewhere or something along those lines.
In my case, I don't think that it's to do with opening too many pages, or pages with lots of content. I'm pretty sure that I've had it happen when opening just 1 or 2 pages in the past.
Today I've opened 5 pages at once with no problems at all.