You may also receive an email from ATT about getting close to your limit and/or hitting the limit and getting charged an extra 15 for more data plan.
Just remember that the amount that you see on the phone and website may not be your complete total. I had an email stating that I was close to my data limit, I kept looking at it and then BAM!!! I was hit with another $15 the night before the new month started. I called and said that I kept an eye on it and they gave me credit. They gave me credit because I did not go over by that much. Just keep that in mind whenever you look at it.
My daughter was just hit with an extra charge of 15.00. I don't know if she kept an eye on her data usage. Plus she just had a baby so I think she had other things to think about.