Omission of important information

So last week I recieved an apparently 20 gig sim that I would pay r249 for. As I sign onto the app an amount of r 600 reflected as the next payment this month. How did I accumulate this that much usage in less than a week? Why do not told about the additional fees? And why was I not informed that 20 gig is actually 10 gig on the day and 10 at night. So now I must pay so much when I was advised that I would be saving?