This is common in shared hosting, they cram as many sites on one box as they can, this allows them to make a profit. It works the same way with their bandwidth, they buy enough for say 20 sites then split it between 30 or more.
Just because you used the same host doesn't mean you where placed on the same server, and even if it was the same server they could have taken on many clients in the time between your two forums.
The best thing to do in this situation is the let the host know about the problem and have them look into it, if they don't get it fixed in a timely manner look elsewhere for hosting.
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