You still have to look at your data usage in settings. Excess data use is almost ALWAYS a faulty phone or app - Not a carrier problem.
If you have multiple phones look at each phone. If you don't reset your stats with each billing period, you may not know which app or service is using excess data. A reset now will show in a day or two if one app or the entire phone has gone "rouge".
THe culprit on our plan was Spotify. An update changed the default streaming to maximum dpi on android phones, tripling the data usage. We also had an iPhone 5s that stopped connecting to wifi, although it indicated it was. With an inactive SIM card, it was not able to connect to the Internet. It took doing a factory reset TWICE to get it working again.
MY sisters android phone had a weather app that suddenly was using both data and battery to excess.
Loss of wifi has been a repeated problem with Apple phones. The most notorious was the update to iOS 7 completely killed the ability for many iPhone 4s to connect to wifi. For this reason, my first suggestion is to back up to a computer and factory reset the iPhone.
