Hosting Nation doesn't offer unlimited hosting and bandwidth for a couple of reasons; the first of which is that nobody can offer unlimited diskspace or bandwidth. If a company offers unlimited hosting, they're banking on the fact that a few high usage customers won't exceed the total diskspace and bandwidth available on the server - so there will be plenty to go around.
The other reason is that most companies offering unlimited deals have fine print in the terms of service that accounts with "extremely high CPU usage" may be penalized. Essentially, you could use as much space and bandwidth as you wanted, until it started to affect other clients.
What we can do is temporarily upgrade your account during months that you'll need it, or simply upgrade you to a custom plan with the bandwidth and diskspace you need to keep your website running.
Visit http://www.unlimband.com for more information on the Unlimited Bandwidth tomfoolery.