My query is why do I start getting data usage warnings from bt. my 20gb of data started on the 23rd of last month I've used just under 10gb and already had a few warnings I've only got 2 days until my data reloads so why the warnings? Is this a way of bt saving money?
If think it may reset itself on the next main billing date, assuming you have BT Broadband as well. So that may not tie up with the day you added your mobile data.
Also, if it is android, it could be the device itself warning if the phone is set to say 12Gb then you will get warnings before you reach the cap. In addition if you reach this cap it will cut off. Check under the settings a limit has been set.