It is downloading sth on the background. You can do 2 things. A) go to a fast wifi and let the download finish automatically. (this is the easy way) Or if you can't find any wifi b) identify what is being downloaded and stop it. Usually Notifications will appear whenever sth is being downloaded but apps like telegram dont notify you... So you just have to restrict data acess for the application that is intensively using your data. Often you may not know which application... Just go to settings and data usage and there will be a bar showing who's going crazy. Good luck