help!! :)

hey guys i have a question about my internet bandwidth,you see i have a limited bandwidth 100 GB per month and until now ive used 121 GB bandwidth and my internet connection is going superb ,usually when i use more then 100 GB before the month ends my internet connection starts going very slow,do you guys have any idea what is going on?not that im complaining im very happy about this :D but i wanna know what is going on?

That'd be a question to ask your ISP. Maybe they increased your limit? Maybe the decided not to cap your speed after you reached your limit, and instead decided to fine you on your monthly bill for going above your limit. ;)