Data Usage Warning?

[Android Question]

I have a T-Mobile Samsung Galaxy S2.
A few weeks ago, in my notifications bar a triangle with an exclamation point appeared and it says 'Data Usage Warning'. I clicked on it but it all looks really confusing. It was there for a few days, then it went away. It came back a couple days ago and then I started having problems receiving videos and gifs and sometimes pictures. When I receive the message it just says the media wasn't included because of a message size restriction. However, my mom has the same phone and she can receive these things so I'm thinking this has something to do with the Data Usage Warning but I'm not sure?
I also can no longer watch YouTube and Vine videos because they won't load.
Is this because of the Data Usage Warning? Do I have to take my phone to a store to get fixed or something?

 

[Rukbat]

Check your account. You probably have an account with a specific maximum amount of data per month. And you've exceeded it for this month. You "fix" the situation by either using less data or paying for an account that aloows more data.