It may not be hackers. If you stream a lot of video, that eats up your data quite fast. Streaming music not so much but still if you stream music all day that is a different story. Another thing is surfing the net and it’s only going to get worse. If you notice there are a lot more sites that auto stream videos and ads when you visit their site. This keeps chipping away at you data caps if you have a limit. Not knowing exactly what type of cellular plan you have we can only make a few guesses as to why your bill may be higher.