Newly bought data disappearing!

I bought data for my phone and before I could even use it, it was already depleted!! I made sure I have no apps running, upgrades or anything like that running in the background. This has been happening now more than once... friends and family complaining about the same thing! Also where does the unused data go should I not use it before end of the month?? I've still paid for it and yet I cannot use it when I need it? *******!