Hi, which device are you using and which app?
Are you connected to 4G or 3G /H /H+
Was it a live steam or a recorded /catch up streamand how long was the programme?
If it was a recordered stream, it could be that the progression bar had progresed
To the end of the programme, although you only watched 12 minute
Even with a very fast 4G stream, it would seem. Inconceivable to use over 2gb
Have you tested again by watching for say 2 or 5 minutes and checking your usage again?