Hi Woody50,
If you are on an unlimited data plan, your speeds will be slowed down after you hit a certian point (3GB for a 3G/HSPA+ Device or 5GB for an LTE Device). You would have received warnings in the form of a text message 2 prior times before your device is slowed down.
If you are on a mobile share plan or a tiered data plan you don't get slowed down.
Hope this helps!